2018-06-16 19:24:48 -07:00
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## Storage
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### MultiStore
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MultiStore is like a root filesystem of an operating system, except
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all the entries are fully Merkleized. You mount onto a MultiStore
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any number of Stores. Currently only KVStores are supported, but in
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the future we may support more kinds of stores, such as a HeapStore
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or a NDStore for multidimensional storage.
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The MultiStore as well as all mounted stores provide caching (aka
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cache-wrapping) for intermediate state (aka software transactional
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memory) during the execution of transactions. In the case of the
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KVStore, this also works for iterators. For example, after running
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the app's AnteHandler, the MultiStore is cache-wrapped (and each
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store is also cache-wrapped) so that should processing of the
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transaction fail, at least the transaction fees are paid and
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sequence incremented.
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The MultiStore as well as all stores support (or will support)
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historical state pruning and snapshotting and various kinds of
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queries with proofs.
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### KVStore
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Here we'll focus on the IAVLStore, which is a kind of KVStore.
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IAVLStore is a fast balanced dynamic Merkle store that also supports
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iteration, and of course cache-wrapping, state pruning, and various
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queries with proofs, such as proofs of existence, absence, range,
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and so on.
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Here's how you mount them to a MultiStore.
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```go
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mainDB, catDB := dbm.NewMemDB(), dbm.NewMemDB()
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fooKey := sdk.NewKVStoreKey("foo")
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barKey := sdk.NewKVStoreKey("bar")
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catKey := sdk.NewKVStoreKey("cat")
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ms := NewCommitMultiStore(mainDB)
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ms.MountStoreWithDB(fooKey, sdk.StoreTypeIAVL, nil)
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ms.MountStoreWithDB(barKey, sdk.StoreTypeIAVL, nil)
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ms.MountStoreWithDB(catKey, sdk.StoreTypeIAVL, catDB)
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```
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In the example above, all IAVL nodes (inner and leaf) will be stored
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in mainDB with the prefix of "s/k:foo/" and "s/k:bar/" respectively,
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thus sharing the mainDB. All IAVL nodes (inner and leaf) for the
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cat KVStore are stored separately in catDB with the prefix of
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"s/\_/". The "s/k:KEY/" and "s/\_/" prefixes are there to
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disambiguate store items from other items of non-storage concern.
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2018-06-16 20:16:04 -07:00
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##
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Mounting an IAVLStore
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TODO:
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IAVLStore: Fast balanced dynamic Merkle store.
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supports iteration.
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MultiStore: multiple Merkle tree backends in a single store
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allows using Ethereum Patricia Trie and Tendermint IAVL in same app
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Provide caching for intermediate state during execution of blocks and
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transactions (including for iteration)
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Historical state pruning and snapshotting.
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Query proofs (existence, absence, range, etc.) on current and retained
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historical state.
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