Message Flow
This section describes the message flow and the semantics of each message type.
(Details of the exact representation of each message appear in Section
Message Formats.) There are several different sub-protocols depending on the
state of the connection: start-up, query, function call, COPY, and termination.
There are also special provisions for asynchronous operations (including
notification responses and command cancellation), which can occur at any time
after the start-up phase.
Start-up
To begin a session, a frontend opens a connection to the server and sends a startup message. This message includes the names of the user and of the database the user wants to connect to; it also identifies the particular protocol version to be used. (Optionally, the startup message can include additional settings for run-time parameters.) The server then uses this information and the contents of its configuration files (such as qhb_hba.conf) to determine whether the connection is provisionally acceptable, and what additional authentication is required (if any).
The server then sends an appropriate authentication request message, to which the frontend must reply with an appropriate authentication response message (such as a password). For all authentication methods except GSSAPI and SASL, there is at most one request and one response. In some methods, no response at all is needed from the frontend, and so no authentication request occurs. For GSSAPI and SASL, multiple exchanges of packets may be needed to complete the authentication.
The authentication cycle ends with the server either rejecting the connection attempt (ErrorResponse), or sending AuthenticationOk.
The possible messages from the server in this phase are:
ErrorResponse
The connection attempt has been rejected. The server then immediately closes the
connection.
AuthenticationOk
The authentication exchange is successfully completed.
AuthenticationKerberosV5
The frontend must now take part in a Kerberos V5 authentication dialog (not
described here, part of the Kerberos specification) with the server. If this is
successful, the server responds with an AuthenticationOk, otherwise it responds
with an ErrorResponse. This is no longer supported.
AuthenticationCleartextPassword
The frontend must now send a PasswordMessage containing the password in clear-text
form. If this is the correct password, the server responds with an AuthenticationOk,
otherwise it responds with an ErrorResponse.
AuthenticationMD5Password
The frontend must now send a PasswordMessage containing the password (with user
name) encrypted via MD5, then encrypted again using the 4-byte random salt
specified in the AuthenticationMD5Password message. If this is the correct
password, the server responds with an AuthenticationOk, otherwise it responds
with an ErrorResponse. The actual PasswordMessage can be computed in SQL as
concat('md5', md5(concat(md5(concat(password, username)), random-salt))). (Keep
in mind the md5() function returns its result as a hex string.)
AuthenticationGSS The frontend must now initiate a GSSAPI negotiation. The frontend will send a GSSResponse message with the first part of the GSSAPI data stream in response to this. If further messages are needed, the server will respond with AuthenticationGSSContinue.
AuthenticationGSSContinue
This message contains the response data from the previous step of GSSAPI negotiation
(AuthenticationGSS, AuthenticationSSPI or a previous AuthenticationGSSContinue).
If the GSSAPI data in this message indicates more data is needed to complete the
authentication, the frontend must send that data as another GSSResponse message.
If GSSAPI authentication is completed by this message, the server will next send
AuthenticationOk to indicate successful authentication or ErrorResponse to
indicate failure.
AuthenticationSASL
The frontend must now initiate a SASL negotiation, using one of the SASL
mechanisms listed in the message. The frontend will send a SASLInitialResponse
with the name of the selected mechanism, and the first part of the SASL data
stream in response to this. If further messages are needed, the server will
respond with AuthenticationSASLContinue. See Section SASL Authentication.
AuthenticationSASLContinue
This message contains challenge data from the previous step of SASL negotiation
(AuthenticationSASL, or a previous AuthenticationSASLContinue). The frontend must
respond with a SASLResponse message.
AuthenticationSASLFinal
SASL authentication has completed with additional mechanism-specific data for
the client. The server will next send AuthenticationOk to indicate successful
authentication, or an ErrorResponse to indicate failure. This message is sent
only if the SASL mechanism specifies additional data to be sent from server to
client at completion.
NegotiateProtocolVersion
The server does not support the minor protocol version requested by the client,
but does support an earlier version of the protocol; this message indicates the
highest supported minor version. This message will also be sent if the client
requested unsupported protocol options (i.e., beginning with _pq_.) in the
startup packet. This message will be followed by an ErrorResponse or a message
indicating the success or failure of authentication.
If the frontend does not support the authentication method requested by the server, then it should immediately close the connection.
After having received AuthenticationOk, the frontend must wait for further messages from the server. In this phase a backend process is being started, and the frontend is just an interested bystander. It is still possible for the startup attempt to fail (ErrorResponse) or the server to decline support for the requested minor protocol version (NegotiateProtocolVersion), but in the normal case the backend will send some ParameterStatus messages, BackendKeyData, and finally ReadyForQuery.
During this phase the backend will attempt to apply any additional run-time parameter settings that were given in the startup message. If successful, these values become session defaults. An error causes ErrorResponse and exit.
The possible messages from the backend in this phase are:
BackendKeyData
This message provides secret-key data that the frontend must save if it wants to
be able to issue cancel requests later. The frontend should not respond to this
message, but should continue listening for a ReadyForQuery message.
ParameterStatus
This message informs the frontend about the current (initial) setting of backend
parameters, such as client_encoding or DateStyle. The frontend can
ignore this message, or record the settings for its future use; see Section
Asynchronous Operations for more details. The frontend should not respond to
this message, but should continue listening for a ReadyForQuery message.
ReadyForQuery
Start-up is completed. The frontend can now issue commands.
ErrorResponse
Start-up failed. The connection is closed after sending this message.
NoticeResponse
A warning message has been issued. The frontend should display the message but
continue listening for ReadyForQuery or ErrorResponse.
The ReadyForQuery message is the same one that the backend will issue after each command cycle. Depending on the coding needs of the frontend, it is reasonable to consider ReadyForQuery as starting a command cycle, or to consider ReadyForQuery as ending the start-up phase and each subsequent command cycle.
Simple Query
A simple query cycle is initiated by the frontend sending a Query message to the backend. The message includes an SQL command (or commands) expressed as a text string. The backend then sends one or more response messages depending on the contents of the query command string, and finally a ReadyForQuery response message. ReadyForQuery informs the frontend that it can safely send a new command. (It is not actually necessary for the frontend to wait for ReadyForQuery before issuing another command, but the frontend must then take responsibility for figuring out what happens if the earlier command fails and already-issued later commands succeed.)
The possible response messages from the backend are:
CommandComplete
An SQL command completed normally.
CopyInResponse
The backend is ready to copy data from the frontend to a table; see Section
COPY Operations.
CopyOutResponse
The backend is ready to copy data from a table to the frontend; see Section
COPY Operations.
RowDescription
Indicates that rows are about to be returned in response to a SELECT, FETCH,
etc. query. The contents of this message describe the column layout of the rows.
This will be followed by a DataRow message for each row being returned to the
frontend.
DataRow
One of the set of rows returned by a SELECT, FETCH, etc. query.
EmptyQueryResponse
An empty query string was recognized.
ErrorResponse
An error has occurred.
ReadyForQuery
Processing of the query string is complete. A separate message is sent to
indicate this because the query string might contain multiple SQL commands.
(CommandComplete marks the end of processing one SQL command, not the whole
string.) ReadyForQuery will always be sent, whether processing terminates
successfully or with an error.
NoticeResponse
A warning message has been issued in relation to the query. Notices are in
addition to other responses, i.e., the backend will continue processing the
command.
The response to a SELECT query (or other queries that return row sets, such as
EXPLAIN or SHOW) normally consists of RowDescription, zero or more DataRow
messages, and then CommandComplete. COPY to or from the frontend invokes
special protocol as described in Section COPY Operations. All other query
types normally produce only a CommandComplete message.
Since a query string could contain several queries (separated by semicolons), there might be several such response sequences before the backend finishes processing the query string. ReadyForQuery is issued when the entire string has been processed and the backend is ready to accept a new query string.
If a completely empty (no contents other than whitespace) query string is received, the response is EmptyQueryResponse followed by ReadyForQuery.
In the event of an error, ErrorResponse is issued followed by ReadyForQuery. All further processing of the query string is aborted by ErrorResponse (even if more queries remained in it). Note that this might occur partway through the sequence of messages generated by an individual query.
In simple Query mode, the format of retrieved values is always text, except when
the given command is a FETCH from a cursor declared with the BINARY option.
In that case, the retrieved values are in binary format. The format codes given
in the RowDescription message tell which format is being used.
A frontend must be prepared to accept ErrorResponse and NoticeResponse messages whenever it is expecting any other type of message. See also Section Asynchronous Operations concerning messages that the backend might generate due to outside events.
Recommended practice is to code frontends in a state-machine style that will accept any message type at any time that it could make sense, rather than wiring in assumptions about the exact sequence of messages.
Multiple Statements In A Simple Query
When a simple Query message contains more than one SQL statement (separated by semicolons), those statements are executed as a single transaction, unless explicit transaction control commands are included to force a different behavior. For example, if the message contains
INSERT INTO mytable VALUES(1);
SELECT 1/0;
INSERT INTO mytable VALUES(2);
then the divide-by-zero failure in the SELECT will force rollback of the first
INSERT. Furthermore, because execution of the message is abandoned at the first
error, the second INSERT is never attempted at all.
If instead the message contains
BEGIN;
INSERT INTO mytable VALUES(1);
COMMIT;
INSERT INTO mytable VALUES(2);
SELECT 1/0;
then the first INSERT is committed by the explicit COMMIT command. The second
INSERT and the SELECT are still treated as a single transaction, so that the
divide-by-zero failure will roll back the second INSERT, but not the first one.
This behavior is implemented by running the statements in a multi-statement Query message in an implicit transaction block unless there is some explicit transaction block for them to run in. The main difference between an implicit transaction block and a regular one is that an implicit block is closed automatically at the end of the Query message, either by an implicit commit if there was no error, or an implicit rollback if there was an error. This is similar to the implicit commit or rollback that happens for a statement executed by itself (when not in a transaction block).
If the session is already in a transaction block, as a result of a BEGIN in
some previous message, then the Query message simply continues that transaction
block, whether the message contains one statement or several. However, if the
Query message contains a COMMIT or ROLLBACK closing the existing transaction
block, then any following statements are executed in an implicit transaction
block. Conversely, if a BEGIN appears in a multi-statement Query message, then
it starts a regular transaction block that will only be terminated by an explicit
COMMIT or ROLLBACK, whether that appears in this Query message or a later one.
If the BEGIN follows some statements that were executed as an implicit transaction
block, those statements are not immediately committed; in effect, they are
retroactively included into the new regular transaction block.
A COMMIT or ROLLBACK appearing in an implicit transaction block is executed
as normal, closing the implicit block; however, a warning will be issued since a
COMMIT or ROLLBACK without a previous BEGIN might represent a mistake. If
more statements follow, a new implicit transaction block will be started for them.
Savepoints are not allowed in an implicit transaction block, since they would conflict with the behavior of automatically closing the block upon any error.
Remember that, regardless of any transaction control commands that may be present, execution of the Query message stops at the first error. Thus for example given
BEGIN;
SELECT 1/0;
ROLLBACK;
in a single Query message, the session will be left inside a failed regular
transaction block, since the ROLLBACK is not reached after the divide-by-zero
error. Another ROLLBACK will be needed to restore the session to a usable state.
Another behavior of note is that initial lexical and syntactic analysis is done on the entire query string before any of it is executed. Thus simple errors (such as a misspelled keyword) in later statements can prevent execution of any of the statements. This is normally invisible to users since the statements would all roll back anyway when done as an implicit transaction block. However, it can be visible when attempting to do multiple transactions within a multi-statement Query. For instance, if a typo turned our previous example into
BEGIN;
INSERT INTO mytable VALUES(1);
COMMIT;
INSERT INTO mytable VALUES(2);
SELCT 1/0;
then none of the statements would get run, resulting in the visible difference
that the first INSERT is not committed. Errors detected at semantic analysis
or later, such as a misspelled table or column name, do not have this effect.
Lastly, note that all the statements within the Query message will observe the same value of statement_timestamp(), since that timestamp is updated only upon receipt of the Query message. This will result in them all observing the same value of transaction_timestamp() as well, except in cases where the query string ends a previously-started transaction and begins a new one.
Extended Query
The extended query protocol breaks down the above-described simple query protocol into multiple steps. The results of preparatory steps can be re-used multiple times for improved efficiency. Furthermore, additional features are available, such as the possibility of supplying data values as separate parameters instead of having to insert them directly into a query string.
In the extended protocol, the frontend first sends a Parse message, which contains a textual query string, optionally some information about data types of parameter placeholders, and the name of a destination prepared-statement object (an empty string selects the unnamed prepared statement). The response is either ParseComplete or ErrorResponse. Parameter data types can be specified by OID; if not given, the parser attempts to infer the data types in the same way as it would do for untyped literal string constants.
Note
A parameter data type can be left unspecified by setting it to zero, or by making the array of parameter type OIDs shorter than the number of parameter symbols ($n) used in the query string. Another special case is that a parameter's type can be specified as void (that is, the OID of the void pseudo-type). This is meant to allow parameter symbols to be used for function parameters that are actually OUT parameters. Ordinarily there is no context in which a void parameter could be used, but if such a parameter symbol appears in a function's parameter list, it is effectively ignored. For example, a function call such as foo($1,$2,$3,$4) could match a function with two IN and two OUT arguments, if $3 and $4 are specified as having type void.
Note
The query string contained in a Parse message cannot include more than one SQL statement; else a syntax error is reported. This restriction does not exist in the simple-query protocol, but it does exist in the extended protocol, because allowing prepared statements or portals to contain multiple commands would complicate the protocol unduly.
If successfully created, a named prepared-statement object lasts till the end of
the current session, unless explicitly destroyed. An unnamed prepared statement
lasts only until the next Parse statement specifying the unnamed statement as
destination is issued. (Note that a simple Query message also destroys the unnamed
statement.) Named prepared statements must be explicitly closed before they can
be redefined by another Parse message, but this is not required for the unnamed
statement. Named prepared statements can also be created and accessed at the SQL
command level, using PREPARE and EXECUTE.
Once a prepared statement exists, it can be readied for execution using a Bind message. The Bind message gives the name of the source prepared statement (empty string denotes the unnamed prepared statement), the name of the destination portal (empty string denotes the unnamed portal), and the values to use for any parameter placeholders present in the prepared statement. The supplied parameter set must match those needed by the prepared statement. (If you declared any void parameters in the Parse message, pass NULL values for them in the Bind message.) Bind also specifies the format to use for any data returned by the query; the format can be specified overall, or per-column. The response is either BindComplete or ErrorResponse.
Note
The choice between text and binary output is determined by the format codes given in Bind, regardless of the SQL command involved. The BINARY attribute in cursor declarations is irrelevant when using extended query protocol.
Query planning typically occurs when the Bind message is processed. If the prepared statement has no parameters, or is executed repeatedly, the server might save the created plan and re-use it during subsequent Bind messages for the same prepared statement. However, it will do so only if it finds that a generic plan can be created that is not much less efficient than a plan that depends on the specific parameter values supplied. This happens transparently so far as the protocol is concerned.
If successfully created, a named portal object lasts till the end of the current
transaction, unless explicitly destroyed. An unnamed portal is destroyed at the
end of the transaction, or as soon as the next Bind statement specifying the
unnamed portal as destination is issued. (Note that a simple Query message also
destroys the unnamed portal.) Named portals must be explicitly closed before they
can be redefined by another Bind message, but this is not required for the unnamed
portal. Named portals can also be created and accessed at the SQL command level,
using DECLARE CURSOR and FETCH.
Once a portal exists, it can be executed using an Execute message. The Execute message specifies the portal name (empty string denotes the unnamed portal) and a maximum result-row count (zero meaning “fetch all rows”). The result-row count is only meaningful for portals containing commands that return row sets; in other cases the command is always executed to completion, and the row count is ignored. The possible responses to Execute are the same as those described above for queries issued via simple query protocol, except that Execute doesn't cause ReadyForQuery or RowDescription to be issued.
If Execute terminates before completing the execution of a portal (due to reaching a nonzero result-row count), it will send a PortalSuspended message; the appearance of this message tells the frontend that another Execute should be issued against the same portal to complete the operation. The CommandComplete message indicating completion of the source SQL command is not sent until the portal's execution is completed. Therefore, an Execute phase is always terminated by the appearance of exactly one of these messages: CommandComplete, EmptyQueryResponse (if the portal was created from an empty query string), ErrorResponse, or PortalSuspended.
At completion of each series of extended-query messages, the frontend should issue
a Sync message. This parameterless message causes the backend to close the current
transaction if it's not inside a BEGIN/COMMIT transaction block (“close”
meaning to commit if no error, or roll back if error). Then a ReadyForQuery
response is issued. The purpose of Sync is to provide a resynchronization point
for error recovery. When an error is detected while processing any extended-query
message, the backend issues ErrorResponse, then reads and discards messages until
a Sync is reached, then issues ReadyForQuery and returns to normal message
processing. (But note that no skipping occurs if an error is detected while
processing Sync — this ensures that there is one and only one ReadyForQuery sent
for each Sync.)
Note
Sync does not cause a transaction block opened withBEGINto be closed. It is possible to detect this situation since the ReadyForQuery message includes transaction status information.
In addition to these fundamental, required operations, there are several optional operations that can be used with extended-query protocol.
The Describe message (portal variant) specifies the name of an existing portal (or an empty string for the unnamed portal). The response is a RowDescription message describing the rows that will be returned by executing the portal; or a NoData message if the portal does not contain a query that will return rows; or ErrorResponse if there is no such portal.
The Describe message (statement variant) specifies the name of an existing prepared statement (or an empty string for the unnamed prepared statement). The response is a ParameterDescription message describing the parameters needed by the statement, followed by a RowDescription message describing the rows that will be returned when the statement is eventually executed (or a NoData message if the statement will not return rows). ErrorResponse is issued if there is no such prepared statement. Note that since Bind has not yet been issued, the formats to be used for returned columns are not yet known to the backend; the format code fields in the RowDescription message will be zeroes in this case.
Tip
In most scenarios the frontend should issue one or the other variant of Describe before issuing Execute, to ensure that it knows how to interpret the results it will get back.
The Close message closes an existing prepared statement or portal and releases resources. It is not an error to issue Close against a nonexistent statement or portal name. The response is normally CloseComplete, but could be ErrorResponse if some difficulty is encountered while releasing resources. Note that closing a prepared statement implicitly closes any open portals that were constructed from that statement.
The Flush message does not cause any specific output to be generated, but forces the backend to deliver any data pending in its output buffers. A Flush must be sent after any extended-query command except Sync, if the frontend wishes to examine the results of that command before issuing more commands. Without Flush, messages returned by the backend will be combined into the minimum possible number of packets to minimize network overhead.
Note
The simple Query message is approximately equivalent to the series Parse, Bind, portal Describe, Execute, Close, Sync, using the unnamed prepared statement and portal objects and no parameters. One difference is that it will accept multiple SQL statements in the query string, automatically performing the bind/ describe/execute sequence for each one in succession. Another difference is that it will not return ParseComplete, BindComplete, CloseComplete, or NoData messages.
Pipelining
Use of the extended query protocol allows pipelining, which means sending a series of queries without waiting for earlier ones to complete. This reduces the number of network round trips needed to complete a given series of operations. However, the user must carefully consider the required behavior if one of the steps fails, since later queries will already be in flight to the server.
One way to deal with that is to make the whole query series be a single
transaction, that is wrap it in BEGIN ... COMMIT. However, this does not help
if one wishes for some of the commands to commit independently of others.
The extended query protocol provides another way to manage this concern, which
is to omit sending Sync messages between steps that are dependent. Since, after
an error, the backend will skip command messages until it finds Sync, this allows
later commands in a pipeline to be skipped automatically when an earlier one
fails, without the client having to manage that explicitly with BEGIN and
COMMIT. Independently-committable segments of the pipeline can be separated by
Sync messages.
If the client has not issued an explicit BEGIN, then each Sync ordinarily causes
an implicit COMMIT if the preceding step(s) succeeded, or an implicit ROLLBACK
if they failed. However, there are a few DDL commands (such as CREATE DATABASE)
that cannot be executed inside a transaction block. If one of these is executed
in a pipeline, it will fail unless it is the first command in the pipeline.
Furthermore, upon success it will force an immediate commit to preserve database
consistency. Thus a Sync immediately following one of these commands has no effect
except to respond with ReadyForQuery.
When using this method, completion of the pipeline must be determined by counting ReadyForQuery messages and waiting for that to reach the number of Syncs sent. Counting command completion responses is unreliable, since some of the commands may be skipped and thus not produce a completion message.
Function Call
The Function Call sub-protocol allows the client to request a direct call of any function that exists in the database's pg_proc system catalog. The client must have execute permission for the function.
Note
The Function Call sub-protocol is a legacy feature that is probably best avoided in new code. Similar results can be accomplished by setting up a prepared statement that doesSELECT function($1, ...). The Function Call cycle can then be replaced with Bind/Execute.
A Function Call cycle is initiated by the frontend sending a FunctionCall message to the backend. The backend then sends one or more response messages depending on the results of the function call, and finally a ReadyForQuery response message. ReadyForQuery informs the frontend that it can safely send a new query or function call.
The possible response messages from the backend are:
ErrorResponse
An error has occurred.
FunctionCallResponse
The function call was completed and returned the result given in the message.
(Note that the Function Call protocol can only handle a single scalar result,
not a row type or set of results.)
ReadyForQuery
Processing of the function call is complete. ReadyForQuery will always be sent,
whether processing terminates successfully or with an error.
NoticeResponse
A warning message has been issued in relation to the function call. Notices are
in addition to other responses, i.e., the backend will continue processing the
command.
COPY Operations
The COPY command allows high-speed bulk data transfer to or from the server.
Copy-in and copy-out operations each switch the connection into a distinct
sub-protocol, which lasts until the operation is completed.
Copy-in mode (data transfer to the server) is initiated when the backend executes
a COPY FROM STDIN SQL statement. The backend sends a CopyInResponse message to
the frontend. The frontend should then send zero or more CopyData messages,
forming a stream of input data. (The message boundaries are not required to have
anything to do with row boundaries, although that is often a reasonable choice.)
The frontend can terminate the copy-in mode by sending either a CopyDone message
(allowing successful termination) or a CopyFail message (which will cause the
COPY SQL statement to fail with an error). The backend then reverts to the
command-processing mode it was in before the COPY started, which will be either
simple or extended query protocol. It will next send either CommandComplete (if
successful) or ErrorResponse (if not).
In the event of a backend-detected error during copy-in mode (including receipt
of a CopyFail message), the backend will issue an ErrorResponse message. If the
COPY command was issued via an extended-query message, the backend will now
discard frontend messages until a Sync message is received, then it will issue
ReadyForQuery and return to normal processing. If the COPY command was issued
in a simple Query message, the rest of that message is discarded and ReadyForQuery
is issued. In either case, any subsequent CopyData, CopyDone, or CopyFail messages
issued by the frontend will simply be dropped.
The backend will ignore Flush and Sync messages received during copy-in mode.
Receipt of any other non-copy message type constitutes an error that will abort
the copy-in state as described above. (The exception for Flush and Sync is for the
convenience of client libraries that always send Flush or Sync after an Execute
message, without checking whether the command to be executed is a COPY FROM STDIN.)
Copy-out mode (data transfer from the server) is initiated when the backend
executes a COPY TO STDOUT SQL statement. The backend sends a CopyOutResponse
message to the frontend, followed by zero or more CopyData messages (always one
per row), followed by CopyDone. The backend then reverts to the command-processing
mode it was in before the COPY started, and sends CommandComplete. The frontend
cannot abort the transfer (except by closing the connection or issuing a Cancel
request), but it can discard unwanted CopyData and CopyDone messages.
In the event of a backend-detected error during copy-out mode, the backend will issue an ErrorResponse message and revert to normal processing. The frontend should treat receipt of ErrorResponse as terminating the copy-out mode.
It is possible for NoticeResponse and ParameterStatus messages to be interspersed between CopyData messages; frontends must handle these cases, and should be prepared for other asynchronous message types as well (see Section Asynchronous Operations). Otherwise, any message type other than CopyData or CopyDone may be treated as terminating copy-out mode.
There is another Copy-related mode called copy-both, which allows high-speed bulk
data transfer to and from the server. Copy-both mode is initiated when a backend
in walsender mode executes a START_REPLICATION statement. The backend sends a
CopyBothResponse message to the frontend. Both the backend and the frontend may
then send CopyData messages until either end sends a CopyDone message. After the
client sends a CopyDone message, the connection goes from copy-both mode to
copy-out mode, and the client may not send any more CopyData messages. Similarly,
when the server sends a CopyDone message, the connection goes into copy-in mode,
and the server may not send any more CopyData messages. After both sides have
sent a CopyDone message, the copy mode is terminated, and the backend reverts to
the command-processing mode. In the event of a backend-detected error during
copy-both mode, the backend will issue an ErrorResponse message, discard frontend
messages until a Sync message is received, and then issue ReadyForQuery and
return to normal processing. The frontend should treat receipt of ErrorResponse
as terminating the copy in both directions; no CopyDone should be sent in this
case. See Section Streaming Replication Protocol for more information on the
subprotocol transmitted over copy-both mode.
The CopyInResponse, CopyOutResponse and CopyBothResponse messages include fields
that inform the frontend of the number of columns per row and the format codes
being used for each column. (As of the present implementation, all columns in a
given COPY operation will use the same format, but the message design does not
assume this.)
Asynchronous Operations
There are several cases in which the backend will send messages that are not specifically prompted by the frontend's command stream. Frontends must be prepared to deal with these messages at any time, even when not engaged in a query. At minimum, one should check for these cases before beginning to read a query response.
It is possible for NoticeResponse messages to be generated due to outside activity; for example, if the database administrator commands a “fast” database shutdown, the backend will send a NoticeResponse indicating this fact before closing the connection. Accordingly, frontends should always be prepared to accept and display NoticeResponse messages, even when the connection is nominally idle.
ParameterStatus messages will be generated whenever the active value changes for
any of the parameters the backend believes the frontend should know about. Most
commonly this occurs in response to a SET SQL command executed by the frontend,
and this case is effectively synchronous — but it is also possible for parameter
status changes to occur because the administrator changed a configuration file
and then sent the SIGHUP signal to the server. Also, if a SET command is rolled
back, an appropriate ParameterStatus message will be generated to report the
current effective value.
At present there is a hard-wired set of parameters for which ParameterStatus will be generated. They are:
application_name
client_encoding
DateStyle
default_transaction_read_only
in_hot_standby
integer_datetimes
IntervalStyle
is_superuser
scram_iterations
server_encoding
server_version
session_authorization
standard_conforming_strings
TimeZone
(default_transaction_read_only and in_hot_standby were not reported by releases before 1.5.0; scram_iterations was not reported by releases before 1.5.3.) Note that server_version, server_encoding and integer_datetimes are pseudo-parameters that cannot change after startup. This set might change in the future, or even become configurable. Accordingly, a frontend should simply ignore ParameterStatus for parameters that it does not understand or care about.
If a frontend issues a LISTEN command, then the backend will send a
NotificationResponse message (not to be confused with NoticeResponse!) whenever
a NOTIFY command is executed for the same channel name.
Note
At present, NotificationResponse can only be sent outside a transaction, and thus it will not occur in the middle of a command-response series, though it might occur just before ReadyForQuery. It is unwise to design frontend logic that assumes that, however. Good practice is to be able to accept NotificationResponse at any point in the protocol.
Canceling Requests in Progress
During the processing of a query, the frontend might request cancellation of the query. The cancel request is not sent directly on the open connection to the backend for reasons of implementation efficiency: we don't want to have the backend constantly checking for new input from the frontend during query processing. Cancel requests should be relatively infrequent, so we make them slightly cumbersome in order to avoid a penalty in the normal case.
To issue a cancel request, the frontend opens a new connection to the server and sends a CancelRequest message, rather than the StartupMessage message that would ordinarily be sent across a new connection. The server will process this request and then close the connection. For security reasons, no direct reply is made to the cancel request message.
A CancelRequest message will be ignored unless it contains the same key data (PID and secret key) passed to the frontend during connection start-up. If the request matches the PID and secret key for a currently executing backend, the processing of the current query is aborted. (In the existing implementation, this is done by sending a special signal to the backend process that is processing the query.)
The cancellation signal might or might not have any effect — for example, if it arrives after the backend has finished processing the query, then it will have no effect. If the cancellation is effective, it results in the current command being terminated early with an error message.
The upshot of all this is that for reasons of both security and efficiency, the frontend has no direct way to tell whether a cancel request has succeeded. It must continue to wait for the backend to respond to the query. Issuing a cancel simply improves the odds that the current query will finish soon, and improves the odds that it will fail with an error message instead of succeeding.
Since the cancel request is sent across a new connection to the server and not across the regular frontend/backend communication link, it is possible for the cancel request to be issued by any process, not just the frontend whose query is to be canceled. This might provide additional flexibility when building multiple- process applications. It also introduces a security risk, in that unauthorized persons might try to cancel queries. The security risk is addressed by requiring a dynamically generated secret key to be supplied in cancel requests.
Termination
The normal, graceful termination procedure is that the frontend sends a Terminate message and immediately closes the connection. On receipt of this message, the backend closes the connection and terminates.
In rare cases (such as an administrator-commanded database shutdown) the backend might disconnect without any frontend request to do so. In such cases the backend will attempt to send an error or notice message giving the reason for the disconnection before it closes the connection.
Other termination scenarios arise from various failure cases, such as core dump at one end or the other, loss of the communications link, loss of message-boundary synchronization, etc. If either frontend or backend sees an unexpected closure of the connection, it should clean up and terminate. The frontend has the option of launching a new backend by recontacting the server if it doesn't want to terminate itself. Closing the connection is also advisable if an unrecognizable message type is received, since this probably indicates loss of message-boundary sync.
For either normal or abnormal termination, any open transaction is rolled back,
not committed. One should note however that if a frontend disconnects while a
non-SELECT query is being processed, the backend will probably finish the query
before noticing the disconnection. If the query is outside any transaction block
(BEGIN ... COMMIT sequence) then its results might be committed before the
disconnection is recognized.
SSL Session Encryption
If QHB was built with SSL support, frontend/backend communications can be encrypted using SSL. This provides communication security in environments where attackers might be able to capture the session traffic. For more information on encrypting QHB sessions with SSL, see Section Secure TCP/IP Connections with SSL.
To initiate an SSL-encrypted connection, the frontend initially sends an SSLRequest message rather than a StartupMessage. The server then responds with a single byte containing S or N, indicating that it is willing or unwilling to perform SSL, respectively. The frontend might close the connection at this point if it is dissatisfied with the response. To continue after S, perform an SSL startup handshake (not described here, part of the SSL specification) with the server. If this is successful, continue with sending the usual StartupMessage. In this case the StartupMessage and all subsequent data will be SSL-encrypted. To continue after N, send the usual StartupMessage and proceed without encryption. (Alternatively, it is permissible to issue a GSSENCRequest message after an N response to try to use GSSAPI encryption instead of SSL.)
The frontend should also be prepared to handle an ErrorMessage response to SSLRequest from the server. The frontend should not display this error message to the user/application, since the server has not been authenticated (CVE-2024-10977). In this case the connection must be closed, but the frontend might choose to open a fresh connection and proceed without requesting SSL.
When SSL encryption can be performed, the server is expected to send only the single S byte and then wait for the frontend to initiate an SSL handshake. If additional bytes are available to read at this point, it likely means that a man-in-the-middle is attempting to perform a buffer-stuffing attack (CVE-2021-23222). Frontends should be coded either to read exactly one byte from the socket before turning the socket over to their SSL library, or to treat it as a protocol violation if they find they have read additional bytes.
An initial SSLRequest can also be used in a connection that is being opened to send a CancelRequest message.
While the protocol itself does not provide a way for the server to force SSL encryption, the administrator can configure the server to reject unencrypted sessions as a byproduct of authentication checking.
GSSAPI Session Encryption
If QHB was built with GSSAPI support, frontend/backend communications can be encrypted using GSSAPI. This provides communication security in environments where attackers might be able to capture the session traffic. For more information on encrypting QHB sessions with GSSAPI, see Section Secure TCP/IP Connections with GSSAPI Encryption.
To initiate a GSSAPI-encrypted connection, the frontend initially sends a GSSENCRequest message rather than a StartupMessage. The server then responds with a single byte containing G or N, indicating that it is willing or unwilling to perform GSSAPI encryption, respectively. The frontend might close the connection at this point if it is dissatisfied with the response. To continue after G, using the GSSAPI C bindings as discussed in RFC 2744 or equivalent, perform a GSSAPI initialization by calling gss_init_sec_context() in a loop and sending the result to the server, starting with an empty input and then with each result from the server, until it returns no output. When sending the results of gss_init_sec_context() to the server, prepend the length of the message as a four byte integer in network byte order. To continue after N, send the usual StartupMessage and proceed without encryption. (Alternatively, it is permissible to issue an SSLRequest message after an N response to try to use SSL encryption instead of GSSAPI.)
The frontend should also be prepared to handle an ErrorMessage response to GSSENCRequest from the server. The frontend should not display this error message to the user/application, since the server has not been authenticated (CVE-2024-10977). In this case the connection must be closed, but the frontend might choose to open a fresh connection and proceed without requesting GSSAPI encryption.
When GSSAPI encryption can be performed, the server is expected to send only the single G byte and then wait for the frontend to initiate a GSSAPI handshake. If additional bytes are available to read at this point, it likely means that a man-in-the-middle is attempting to perform a buffer-stuffing attack (CVE-2021-23222). Frontends should be coded either to read exactly one byte from the socket before turning the socket over to their GSSAPI library, or to treat it as a protocol violation if they find they have read additional bytes.
An initial GSSENCRequest can also be used in a connection that is being opened to send a CancelRequest message.
Once GSSAPI encryption has been successfully established, use gss_wrap() to encrypt the usual StartupMessage and all subsequent data, prepending the length of the result from gss_wrap() as a four byte integer in network byte order to the actual encrypted payload. Note that the server will only accept encrypted packets from the client which are less than 16kB; gss_wrap_size_limit() should be used by the client to determine the size of the unencrypted message which will fit within this limit and larger messages should be broken up into multiple gss_wrap() calls. Typical segments are 8kB of unencrypted data, resulting in encrypted packets of slightly larger than 8kB but well within the 16kB maximum. The server can be expected to not send encrypted packets of larger than 16kB to the client.
While the protocol itself does not provide a way for the server to force GSSAPI encryption, the administrator can configure the server to reject unencrypted sessions as a byproduct of authentication checking.