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abci: note on concurrency #258

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Feb 26, 2021
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abci: note on concurrency #258

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abci: note on concurrency
ebuchman Feb 13, 2021
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Update spec/abci/apps.md
tac0turtle Feb 26, 2021
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22 changes: 21 additions & 1 deletion spec/abci/apps.md
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Expand Up @@ -22,6 +22,26 @@ Since Tendermint maintains four concurrent ABCI connections, it is typical
for an application to maintain a distinct state for each, and for the states to
be synchronized during `Commit`.

### Concurrency

In principle, each of the four ABCI connections operate concurrently with one
another. This means applications need to ensure access to state is
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Is this tendermint state or application. Would be good to differentiate. These two separate states get conflated often

thread safe. In practice, both the
[default in-process ABCI client](https://github.com/tendermint/tendermint/blob/v0.34.4/abci/client/local_client.go#L18)
and the
[default Go ABCI
server](https://github.com/tendermint/tendermint/blob/v0.34.4/abci/server/socket_server.go#L32)
use global locks across all connections, so they are not
concurrent at all. This means if your app is written in Go, and compiled in-process with Tendermint
using the default `NewLocalClient`, or run out-of-process using the default `SocketServer`,
ABCI messages from all connections will be linearizable (received one at a
time).

The existence of this global mutex means Go application developers can get
thread safety for application state by routing *all* reads and writes through the ABCI
system. Thus it may be *unsafe* to expose application state directly to an RPC
interface, and unless explicit measures are taken, all queries should be routed through the ABCI Query method.

### BeginBlock

The BeginBlock request can be used to run some code at the beginning of
Expand All @@ -37,7 +57,7 @@ pick up from when it restarts. See information on the Handshake, below.
Application state should only be persisted to disk during `Commit`.
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As in contrast to what?


Before `Commit` is called, Tendermint locks and flushes the mempool so that no new messages will
be received on the mempool connection. This provides an opportunity to safely update all three
be received on the mempool connection. This provides an opportunity to safely update all four connection
states to the latest committed state at once.

When `Commit` completes, it unlocks the mempool.
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