Concurrency Limiting

In certain cases it may be useful to ensure that only a specific amount of instances of a command are running at any given time. This can be achieved by setting a concurrency limiter.

GatewayREST

@client.include
# Limit the command to 1 instance per channel
@arc.with_concurrency_limit(arc.channel_concurrency(1))
@arc.slash_command("name", "description")
async def foo(ctx: arc.GatewayContext) -> None:
    await ctx.respond("Hello, I'm running for the next 10 seconds!")
    await asyncio.sleep(10.0)
    await ctx.edit_initial_response("I'm done!")

@foo.set_error_handler
async def foo_error_handler(ctx: arc.GatewayContext, error: Exception) -> None:
    if isinstance(error, arc.MaxConcurrencyReachedError):
        await ctx.respond(
            "Max concurrency reached!"
            f"\nThis command can only have `{error.max_concurrency}` instances running."
        )
    else:
        raise error

@client.include
# Limit the command to 1 instance per channel
@arc.with_concurrency_limit(arc.channel_concurrency(1))
@arc.slash_command("name", "description")
async def foo(ctx: arc.RESTContext) -> None:
    await ctx.respond("Hello, I'm running for the next 10 seconds!")
    await asyncio.sleep(10.0)
    await ctx.edit_initial_response("I'm done!")

@foo.set_error_handler
async def foo_error_handler(ctx: arc.RESTContext, error: Exception) -> None:
    if isinstance(error, arc.MaxConcurrencyReachedError):
        await ctx.respond(
            "Max concurrency reached!"
            f"\nThis command can only have `{error.max_concurrency}` instances running."
        )
    else:
        raise error

Try invoking the above command while it is still running, in the same channel! It should fail and the exception will be caught by the error handler.

There's also other types of concurrency limiters, these are:

• arc.global_concurrency() - Limit concurrent instances of a command globally
• arc.guild_concurrency() - Limit concurrent instances of a command per guild
• arc.channel_concurrency() - Limit concurrent instances of a command per channel
• arc.user_concurrency() - Limit concurrent instances of a command per user
• arc.member_concurrency() - Limit concurrent instances of a command per user per guild
• arc.custom_concurrency() - Limit concurrent instances of a command using a custom key extraction function

Concurrency limiters can be added to commands, command groups, plugins, and even the client using either the @arc.with_concurrency_limit() decorator (as shown above) or via the .set_concurrency_limiter() method.

Tip

You can also define your own concurrency limiter implementations by making them conform to the ConcurrencyLimiterProto protocol.