Large Transactions in TiDB

2020-05-15Nick CameronEngineering

Large transactions in TiDB

TiDB is an open-source, distributed SQL database that supports Hybrid Transactional/Analytical Processing (HTAP) workloads. In TiDB 4.0, we've extended the transaction system to handle large transactions. Previously, TiDB limited the number of reads and writes in a transaction. In version 4.0, there is a much larger size limit on transactions (10 GB). In this blog post, I'll describe how we implemented support for large transactions. This post won't explain TiDB's transactions, I'll have a post about that at some point.

Large transactions caused problems for a few reasons: they take up a lot of memory in TiDB, they keep locks on many keys for a long time, which blocks other transactions from making progress, and they can exceed their time-to-live (TTL) and be rolled-back even though they are still working.

To deal with the memory issues, we have made changes to our in-memory buffers in TiDB, I don't do much work with TiDB, so I'm afraid I can't go into detail.

Solving the issue with transactions timing out is fairly straightforward: the TTL is stored with the primary lock in TiKV, the storage engine for TiDB. (Each lock in a transaction has a reference to the primary). TiDB can send a heartbeat message to TiKV to extend the TTL and keep the transaction alive if necessary.

For the problem of blocking other transactions, we must introduce a new concept - the min_commit_ts, which is the minimum time at which the transaction can be committed. We start a transaction with min_commit_ts = start_ts + 1. When a transaction is committed, if the commit_ts is smaller than the min_commit_ts, then an error is triggered and sent to TiDB. TiDB can then retry committing with a later timestamp.

The clever part happens when another transaction (let's call it txn B) is blocked from reading by the large transaction (txn A). In this case, rather than txn B being blocked, it will update the min_commit_ts of txn A's lock, setting it to the start_ts of txn B + 1. (We can't do this for writes, but that is not too bad since we would always expect writes from one transaction to block writes from another). This is a CheckTxnStatus request, but TiKV adds it to its work queue, directly rather than sending it. The min_commit_ts can also be updated by TiDB sending an explicit CheckTxnStatus request.

The addition of min_commit_ts maintains our snapshot isolation property. For an intuition why, imagine that txn B came after txn A was committed. If txn B's start_ts is less than txn A's commit_ts, we would read the old value (i.e., pre-txn A). By adding the min_commit_ts to txn A and keeping it up to date, we are guaranteeing in advance that txn A will not be committed until after txn B's start_ts, i.e., that reading the old value is valid for txn B.

This post was originally published on Nick Cameron's blog.

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