ceph/src/rocksdb/java/understanding_options.md

   1 # How RocksDB Options and their Java Wrappers Work
   2
   3 Options in RocksDB come in many different flavours. This is an attempt at a taxonomy and explanation.
   4
   5 ## RocksDB Options
   6
   7 Initially, I believe, RocksDB had only database options. I don't know if any of these were mutable. Column families came later. Read on to understand the terminology.
   8
   9 So to begin, one sets up a collection of options and starts/creates a database with these options. That's a useful way to think about it, because from a Java point-of-view (and I didn't realise this initially and got very confused), despite making native calls to C++, the `API`s are just manipulating a native C++ configuration object. This object is just a record of configuration, and it must later be passed to the database (at create or open time) in order to apply the options.
  10
  11 ### Database versus Column Family
  12
  13 The concept of the *column family* or `CF` is widespread within RocksDB. I think of it as a data namespace, but conveniently transactions can operate across these namespaces. The concept of a default column family exists, and when operations do not refer to a particular `CF`, it refers to the default.
  14
  15 We raise this w.r.t. options because many options, perhaps most that users encounter, are *column family options*. That is to say they apply individually to a particular column family, or to the default column family. Crucially also, many/most/all of these same options are exposed as *database options* and then apply as the default for column families which do not have the option set explicitly. Obviously some database options are naturally database-wide; they apply to the operation of the database and don't make any sense applied to a column family.
  16
  17 ### Mutability
  18
  19 There are 2 kinds of options
  20
  21 - Mutable options
  22 - Immutable options. We name these in contrast to the mutable ones, but they are usually referred to unqualified.
  23
  24 Mutable options are those which can be changed on a running `RocksDB` instance. Immutable options can only be configured prior to the start of a database. Of course, we can configure the immutable options at this time too; The entirety of options is a strict superset of the mutable options.
  25
  26 Mutable options (whether column-family specific or database-wide) are manipulated at runtime with builders, so we have `MutableDBOptions.MutableDBOptionsBuilder` and `MutableColumnFamilyOptions.MutableColumnFamilyOptionsBuilder` which share tooling classes/hierarchy and maintain and manipulate the relevant options as a `(key,value)` map.
  27
  28 Mutable options are then passed using `setOptions()` and `setDBOptions()` methods on the live RocksDB, and then take effect immediately (depending on the semantics of the option) on the database.
  29
  30 ### Advanced
  31
  32 There are 2 classes of options
  33
  34 - Advanced options
  35 - Non-advanced options
  36
  37 It's not clear to me what the conceptual distinction is between advanced and not. However, the Java code takes care to reflect it from the underlying C++.
  38
  39 This leads to 2 separate type hierarchies within column family options, one for each `class` of options. The `kind`s are represented by where the options appear in their hierarchy.
  40
  41 ```java
  42 interface ColumnFamilyOptionsInterface<T extends ColumnFamilyOptionsInterface<T>>
  43     extends AdvancedColumnFamilyOptionsInterface<T>
  44 interface MutableColumnFamilyOptionsInterface<T extends MutableColumnFamilyOptionsInterface<T>>
  45     extends AdvancedMutableColumnFamilyOptionsInterface<T>
  46 ```
  47
  48 And then there is ultimately a single concrete implementation class for CF options:
  49
  50 ```java
  51 class ColumnFamilyOptions extends RocksObject
  52     implements ColumnFamilyOptionsInterface<ColumnFamilyOptions>,
  53     MutableColumnFamilyOptionsInterface<ColumnFamilyOptions>
  54 ```
  55
  56 as there is a single concrete implementation class for DB options:
  57
  58 ```java
  59 class DBOptions extends RocksObject
  60     implements DBOptionsInterface<DBOptions>,
  61     MutableDBOptionsInterface<DBOptions>
  62 ```
  63
  64 Interestingly `DBOptionsInterface` doesn't extend `MutableDBOptionsInterface`, if only in order to disrupt our belief in consistent basic laws of the Universe.
  65
  66 ## Startup/Creation Options
  67
  68 ```java
  69 class Options extends RocksObject
  70     implements DBOptionsInterface<Options>,
  71     MutableDBOptionsInterface<Options>,
  72     ColumnFamilyOptionsInterface<Options>,
  73     MutableColumnFamilyOptionsInterface<Options>
  74 ```
  75
  76 ### Example - Blob Options
  77
  78 The `enable_blob_files` and `min_blob_size` options are per-column-family, and are mutable. The options also appear in the unqualified database options. So by initial configuration, we can set up a RocksDB database where for every `(key,value)` with a value of size at least `min_blob_size`, the value is written (indirected) to a blob file. Blobs may share a blob file, subject to the configuration values set. Later, using the `MutableColumnFamilyOptionsInterface` of the `ColumnFamilyOptions`, we can choose to turn this off (`enable_blob_files=false`) , or alter the `min_blob_size` for the default column family, or any other column family. It seems to me that we cannot, though, mutate the column family options for all column families using the
  79 `setOptions()` mechanism, either for all existing column families or for all future column families; but maybe we can do the latter on a re-`open()/create()'