Pluggable commands

Starting from epgsql 4.0.0 it's possible to make a custom epgsql commands. By term "command" we assume a single request -> response sequence. Under the hood it might contain many PostgreSQL protocol command requests and responses, but from the point of view of epgsql user, it's a single request that produces a single response. Examples of such commands are connect, squery, equery, prepared_query, parse/bind/execute and so on. See src/commands for a full list and examples. Basically, almost all epgsql end-user APIs are implemented as a commands.

It is possible to send many commands without waiting for a result of previously sent ones (pipelining) by using epgsqla or epgsqli interfaces.

Possible usecases

Why you may want to implement a custom command? Some ideas:

You are not satisfied by performance or functionality of epgsql's built-in commands
To create a version of equery with built-in statement cache
To create a single-roundtrip equery (currently equery works by combining parse and equery commands)
To construct some tricky batching commands, eg, bulk-inserts

This can be done by following steps

If you are not familiar with PostgreSQL wire protocol, please, read at least Message Flow and Message Formats sections of PostgreSQL documentation. But, basicaly, whole Frontend/Backend Protocol would be nice to know.

Implement epgsql_command behaviour callback module

See epgsql_command.

This module should have following functions exported:

init(any()) -> state().

Called only once when command was received and is about to be executed by epgsql connection process. Command's arguments are passed as callback argument, see epgsql_sock:sync_command/3 and epgsql_sock:async_command/4. Should initialize and return command's state that will be passed to all subsequent callbacks. No PostgreSQL interactions should be done here.

execute(pg_sock(), state()) ->
    {ok, pg_sock(), state()}
  | {stop, Reason :: any(), Response :: any(), pg_sock()}.

Client -> Server packets should be sent from this callback by epgsql_sock:send_multi/2 or epgsql_sock:send/3. epgsql_wire module usualy used to create wire protocol packets. Please, notice, that many packets might be sent at once. See epgsql_cmd_equery as an example.

This callback might be executed more than once for a single command execution if your command requires response for some of the packets to send next packet (more than one round-trip). Since epgsql is asynchronous under the hood, you can't just do blocking receive. See handle_message/4 -> {requeue, ...} and epgsql_cmd_connect as an example.

pg_sock() is an opaque state of a epgsql_sock process. There are some APIs to get or set some fields on it in epgsql_sock module.

handle_message(Type :: byte(), Payload :: binary() | query_error(),
               pg_sock(), state()) ->
    {noaction, pg_sock()}
  | {noaction, pg_sock(), state()}
  | {add_row, tuple(), pg_sock(), state()}
  | {add_result, Data :: any(), Notification :: any(), pg_sock(), state()}
  | {finish, Result :: any(), Notification :: any(), pg_sock()}
  | {requeue, pg_sock(), state()}
  | {stop, Reason :: any(), Response :: any(), pg_sock()}
  | {sync_required, Why :: any()}
  | unknown.

Server -> Client packet handling code. Packet Type byte is integer ID of a protocol packet, basicaly the 1st byte of a packet. And Payload is the rest bytes of a packet. epgsql_wire module have some helpers that might help to decode packet payload.

In case when epgsql connection got an error packet from server, it will be decoded and Payload will be query_error() instead of binary.

Please, NEVER call epgsql_sock:send/3/epgsql_sock:send_multi/2 from this callback! Use requeue return instead! Otherwise you will break pipelining!

This callback should return one of the following responses to control command's behaviour:

{noaction, pg_sock()} - to do nothing (this usualy means that packet was ignored)
{noaction, pg_sock(), state()} - do nothing, but update command's state
{add_row, tuple(), pg_sock(), state()} - add a row to current resultset rows accumulator. You may get current accumulated resultset by epgsql_sock::get_rows(pg_sock()) (except when epgsqli interface is used).
{add_result, Result :: any(), Notification :: any(), pg_sock(), state()} - add new result to list of results. Usualy all commands have only single result, except squery when multiple SQL queries was passed separated by a semicolon and execute_batch. You usualy will just return smth like {ok, epgsql_sock:get_rows(PgSock)} or some kind of error as a result. Notification is used for epgsqli interface. You may get current list of accumulated results by epgsql_sock:get_results(pg_sock()).
{finish, Results, Notification, pg_sock(), state()} - returned when command was successfuly executed and no more actions needed. Results will be returned to a client as a result of command execution and command will be descheduled from epgsql connection process. You usualy use result of epgsql_sock:get_results/1 as a Results. Notification is used for epgsqli interface.
{requeue, pg_sock(), state()} - asks epgsql process to put this command to execution queue once again (with a new state). That means that execute/2 callback will be executed again and new packets might be sent from client to server. This way you can implement chatty commands with multiple request -> response sequences. See epgsql_cmd_connect as an example.
{stop, Reason, Response, pg_sock()} - returned when some unrecoverable error occured and you want to terminate epgsql connection process. Response will be returned as a command result and Reason will be process termination reason. Please, try to avoid use of this response if possible.
{sync_required, Why} - returned to finish command execution, flush enqueued but not yet executed commands and to set epgsql process in to sync_required state. In this state it will not accept any commands except epgsql_cmd_sync. This usualy means that multipacket protocol sequence was done out-of-order (eg, bind before parse), so, client and server states are out-of-sync and we need to reset them.
unknown - command got unexpected packet. Connection process will be terminated with {error, {unexpected_message, Type, Payload, state()}}. Usualy returned from a catch-all last clause.

Command now can be executed

By calling

epgsql_sock:sync_command(connection(), command(), Args :: any()) for a gen_server:call-style, synchronous behaviour (epgsql-like API)
epgsql_sock:async_command(connection(), cast, command(), Args :: any()) for asynchronous behaviour when whole resultset will be delivered as a single erlang message (epgsqla-like API)
epgsql_sock:async_command(connection(), incremental, command(), Args :: any()) for asynchronous behaviour when each row and some status info will be delivered as separate erlang messages (epgsqli-like API)

command() is a name of a module, implementing epgsql_command behaviour. Args may be any (eg, SQL query / arguments / options), they will be passed to init/1 callback as is.

Tips

If you are implementing your command outside of a epgsql main tree, it might be handy to add do(Conn, Arg1, Arg2...) -> epgsql_sock:sync_command(Conn, ?MODULE, Args). to incapsulate epgsql_sock calls and provide end-user API.
Don't be afraid of requeue. It might make your code more complex, but will make it possible to implement complex multistep logick inside of a single command
epgsql_sock module have some APIs that might be used from a commands. Refer to module's source code. epgsql_wire have some helpers to encode/decode wire protocol and data packets.
Packet IDs are defined in include/protocol.hrl
Again, never try to send packets from handle_message/4 or init/1 callbacks!
Note that any error in callback functions will cause crash of epgsql connection process!

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