box.session.push() | Tarantool



box.session.push(message[, sync])

Generate an out-of-band message. By “out-of-band” we mean an extra message which supplements what is passed in a network via the usual channels. Although box.session.push() can be called at any time, in practice it is used with networks that are set up with module, and it is invoked by the server (on the “remote database system” to use our terminology for, and the client has options for getting such messages.

This function returns an error if the session is disconnected.

  • message (any-Lua-type) – what to send
  • sync (int) – an optional argument to indicate what the session is, as taken from an earlier call to box.session.sync(). If it is omitted, the default is the current box.session.sync() value.

{nil, error} or true:

  • If the result is an error, then the first part of the return is nil and the second part is the error object.
  • If the result is not an error, then the return is the boolean value true.
  • When the return is true, the message has gone to the network buffer as a packet with the code IPROTO_CHUNK (0x80).

The server’s sole job is to call box.session.push(), there is no automatic mechanism for showing that the message was received.

The client’s job is to check for such messages after it sends something to the server. The major client methods – conn:call, conn:eval, conn:select, conn:insert, conn:replace, conn:update, conn:upsert, delete – may cause the server to send a message.

Situation 1: when the client calls synchronously with the default {async=false} option. There are two optional additional options: on_push=function-name, and on_push_ctx=function-argument. When the client receives an out-of-band message for the session, it invokes “function-name(function-argument)”. For example, with options {on_push=table.insert, on_push_ctx=messages}, the client will insert whatever it receives into a table named ‘messages’.

Situation 2: when the client calls asynchronously with the non-default {async=true} option. Here on_push and on_push_ctx are not allowed, but the messages can be seen by calling pairs() in a loop.

Situation 2 complication: pairs() is subject to timeout. So there is an optional argument = timeout per iteration. If timeout occurs before there is a new message or a final response, there is an error return. To check for an error one can use the first loop parameter (if the loop starts with “for i, message in future:pairs()” then the first loop parameter is i). If it is box.NULL then the second parameter (in our example, “message”) is the error object.


-- Make two shells. On Shell#1 set up a "server", and
-- in it have a function that includes box.session.push:
x = 0
fiber = require('fiber')
function server_function() x=x+1; fiber.sleep(1); box.session.push(x); end

-- On Shell#2 connect to this server as a "client" that
-- can handle Lua (such as another Tarantool server operating
-- as a client), and initialize a table where we'll get messages:
net_box = require('')
conn = net_box.connect(3301)
messages_from_server = {}

-- On Shell#2 remotely call the server function and receive
-- a SYNCHRONOUS out-of-band message:
conn:call('server_function', {},
          {is_async = false,
           on_push = table.insert,
           on_push_ctx = messages_from_server})
-- After a 1-second pause that is caused by the fiber.sleep()
-- request inside server_function, the result in the
--  messages_from_server table will be: 1. Like this:
-- tarantool> messages_from_server
-- ---
-- - - 1
-- ...
-- Good. That shows that box.session.push(x) worked,
-- because we know that x was 1.

-- On Shell#2 remotely call the same server function and
-- get an ASYNCHRONOUS out-of-band message. For this we cannot
-- use on_push and on_push_ctx options, but we can use pairs():
future = conn:call('server_function', {}, {is_async = true})
messages = {}
keys = {}
for i, message in future:pairs() do
    table.insert(messages, message) table.insert(keys, i) end
for i, message in future:pairs() do
    table.insert(messages, message) table.insert(keys, i) end
-- There is no pause because conn:call does not wait for
-- server_function to finish. The first time that we go through
-- the pairs() loop, we see the messages table is empty. Like this:
-- tarantool> messages
-- ---
-- - - 2
--   - []
-- ...
-- That is okay because the server hasn't yet called
-- box.session.push(). The second time that we go through
-- the pairs() loop, we see the value of x at the time of
-- the second call to box.session.push(). Like this:
-- tarantool> messages
-- ---
-- - - 2
--   - &0 []
--   - 2
--   - *0
-- ...
-- Good. That shows that the message was asynchronous, and
-- that box.session.push() did its job.