Pruning the txns collection

[Menno Smits]

On 14 May 2015 at 06:41, Gustavo Niemeyer <gustavo.niemeyer at canonical.com>
wrote:

>
> You are right that it's not that simple, but it's not that complex either
> once you understand the background.
>
> Transactions are applied by the txn package by tagging each one of the
> documents that will participate in the transaction with the transaction id
> they are participating in. When mgo goes to apply a transaction in that
> same document, it will tag the document with the new transaction id, and
> then evaluate all the transactions it is part of. If you drop one of the
> transactions that a document claims to be participating in, then the txn
> package will rightfully complain since it cannot tell the state of a
> transaction that explicitly asked to be considered for the given document.
>
> That means the solution is to make sure removed transactions are 1) in a
> final state; and 2) not being referenced by any tagged documents.
>

Thanks. This explanation clarifies things a lot.


>
> The txn package itself collects garbage from old transactions as new
> transactions are applied, but it doesn't guarantee that right after a
> transaction reaches a final state it will be collected. This can lead to
> pretty old transactions being referenced, if these documents are never
> touched again.
>

I was confused by this part when I read it because I don't see anywhere in
the mgo/txn code where cleanup of the txn collection already occurs. To
summarise our later IRC conversation for anyone who might be interested:
mgo/txn doesn't currently prune the txns collection, but it *does* prune
references to applied transactions from the txn-queue fields on documents.


>
> So, you have two choices to collect these old documents:
>
> 1. Clean up the transaction references from all documents
>
> or
>
> 2. Just make sure the transaction being removed is not referenced anywhere
>
> I would personally go for 2, as it is a read-only operation everywhere but
> in the transactions collection itself, to drop the transaction document.
>

I agree that #2 is preferable and I have a fairly straightforward strategy
in mind to make this happen. I'll work on that today.


Note that the same rules here apply to the stash collection as well.
>

Noted. I know how this hangs together from my work with PurgeMissing.

Thanks,
Menno
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