michalmuskala
PersistentEts - lightweight persistence over ets
Another small library today.
PersistentEts
Hex: persistent_ets | Hex
GitHub: GitHub - michalmuskala/persistent_ets
Ets table backed by a persistence file.
The table is persisted using the :ets.file2tab/2 and :ets.tab2file/3 functions.
Table is to be created with PersistentEts.new/3 in place of :ets.new/2. After that all functions from :ets can be used like with any other table, except :ets.give_away/3 and :ets.delete/1 - replacement functions are provided in this module. The :ets.setopts/2 function to change the heir is not supported - the heir setting is leveraged by the persistence mechanism.
Like with regular ets table, the table is destroyed once the owning process (the one that called PersistentEts.new/3) dies, but the table data is persisted so it will be re-read when table is opened again.
Example
pid = spawn(fn ->
:foo = PersistentEts.new(:foo, "table.tab", [:named_table])
:ets.insert(:foo, [a: 1])
end)
Process.exit(pid, :diediedie)
PersistentEts.new(:foo, "table.tab", [:named_table])
[a: 1] = :ets.tab2list(:foo)
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michalmuskala
With Dets every operation (read or write) hits the disk. For many application such a performance penalty (compared to ets) is not acceptable. Furthermore Dets tables are limited to 2GB. Dets doesn’t support the ordered_set table type either.
With PersistentEts, the table remains in memory, so all read and write operations have the same performance they would have with pure ets. Only periodically the table state is saved to a file. There’s also no file limit, besides the memory and disk limitations. Since it’s a regular Ets table, all types are fully supported.
hubertlepicki
Could you highlight for us how it differs from DETS and why would someone choose one against another?
hubertlepicki
Copy paste that to readme now.
OvermindDL1
Is there a benchmark of it compared to Mnesia with duplicate_bag tables using dirty read/writes (basically ETS that is DETS backed at that point) and similar settings for PersistentEts? 
Does it only persist to disk ‘on occasion’ or after every write? Does it do it when the owner process is terminated? I’m guessing via the file2tab and such that it is serializing out the entire ETS table every write out instead of only the differences?
DanCouper
This is serendipitous; I’m prototyping something at the minute, and this fits the bill exactly. Wanted to have ETS tables that held a specific state for users while they were all connected that could easily be saved for recovery when users came back online (it’s a procedural generation toy, the ETS table provided ‘terrain’ that all users of the toy, and all their controlled processes, can access). Mnesia didn’t quite seem to fit the bill, seemed a bit of a faff, just wanted something brutally simple to get thing running quickly, so thanks for this
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