tendermint/docs/tendermint-core/mempool.md

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# Mempool

## Transaction ordering

Currently, there's no ordering of transactions other than the order they've
arrived (via RPC or from other nodes).

So the only way to specify the order is to send them to a single node.

valA:

- `tx1`
- `tx2`
- `tx3`

If the transactions are split up across different nodes, there's no way to
ensure they are processed in the expected order.

valA:

- `tx1`
- `tx2`

valB:

- `tx3`

If valB is the proposer, the order might be:

- `tx3`
- `tx1`
- `tx2`

If valA is the proposer, the order might be:

- `tx1`
- `tx2`
- `tx3`

That said, if the transactions contain some internal value, like an
order/nonce/sequence number, the application can reject transactions that are
out of order. So if a node receives `tx3`, then `tx1`, it can reject `tx3` and then
accept `tx1`. The sender can then retry sending `tx3`, which should probably be
rejected until the node has seen `tx2`.