RocksDB Iterator

RocksDB Iterator allows users to iterate over the DB forward and backward in a sorted manner. It also has the ability to seek to a specific key inside the DB, to achieve that .. the Iterator need to access the DB as a sorted stream.RocksDB Iterator implementation class is named DBIter, In this wiki page we will discuss how DBIter works and what is it composed of. In the following figure, you can see the design of DBIter and what it's composed of.

Higher quality diagram

DBIter

Implementation: db/db_iter.cc

Interface: Iterator

DBIter is a wrapper around an InternalIterator (In this case a MergingIterator).DBIter's job is to parse InternalKeys exposed by the underlying InternalIterator and expose them as user keys.

Example:

The underlying InternalIterator exposed

InternalKey(user_key="Key1", seqno=10, Type=Put)    | Value = "KEY1_VAL2"
InternalKey(user_key="Key1", seqno=9,  Type=Put)    | Value = "KEY1_VAL1"
InternalKey(user_key="Key2", seqno=16, Type=Put)    | Value = "KEY2_VAL2"
InternalKey(user_key="Key2", seqno=15, Type=Delete) | Value = "KEY2_VAL1"
InternalKey(user_key="Key3", seqno=7,  Type=Delete) | Value = "KEY3_VAL1"
InternalKey(user_key="Key4", seqno=5,  Type=Put)    | Value = "KEY4_VAL1"

But what DBIter will expose to the user is

Key="Key1"  | Value = "KEY1_VAL2"
Key="Key2"  | Value = "KEY2_VAL2"
Key="Key4"  | Value = "KEY4_VAL1"

MergingIterator

Implementation: table/merging_iterator.cc

Interface: InternalIterator

The MergingIterator is composed of many child iterators, MergingIterator is basically a heap for Iterators.In MergingIterator we put all child Iterators in a heap and expose them as one sorted stream.

Example:

The underlying child Iterators exposed

= Child Iterator 1 =
InternalKey(user_key="Key1", seqno=10, Type=Put)    | Value = "KEY1_VAL2"
= Child Iterator 2 =
InternalKey(user_key="Key1", seqno=9,  Type=Put)    | Value = "KEY1_VAL1"
InternalKey(user_key="Key2", seqno=15, Type=Delete) | Value = "KEY2_VAL1"
InternalKey(user_key="Key4", seqno=5,  Type=Put)    | Value = "KEY4_VAL1"
= Child Iterator 3 =
InternalKey(user_key="Key2", seqno=16, Type=Put)    | Value = "KEY2_VAL2"
InternalKey(user_key="Key3", seqno=7,  Type=Delete) | Value = "KEY3_VAL1"

The MergingIterator will keep all child Iterators in a heap and expose them as one sorted stream

InternalKey(user_key="Key1", seqno=10, Type=Put)    | Value = "KEY1_VAL2"
InternalKey(user_key="Key1", seqno=9,  Type=Put)    | Value = "KEY1_VAL1"
InternalKey(user_key="Key2", seqno=16, Type=Put)    | Value = "KEY2_VAL2"
InternalKey(user_key="Key2", seqno=15, Type=Delete) | Value = "KEY2_VAL1"
InternalKey(user_key="Key3", seqno=7,  Type=Delete) | Value = "KEY3_VAL1"
InternalKey(user_key="Key4", seqno=5,  Type=Put)    | Value = "KEY4_VAL1"

MemtableIterator

Implementation: db/memtable.cc

Interface: InternalIterator

This is a wrapper around MemtableRep::Iterator, Every memtable representation implement it's own Iterator to expose the keys/values in the memtable as a sorted stream

BlockIter

Implementation: table/block.h

Interface: InternalIterator

This Iterator is used to read blocks from SST file, whether these blocks were index blocks or data blocks.Since SST file blocks are sorted and immutable, we load the block in memory and create a BlockIter for this sorted data.

TwoLevelIterator

Implementation: table/two_level_iterator.cc

Interface: InternalIterator

A TwoLevelIterator is composed of 2 Iterators

• First level Iterator (firstlevel_iter)
• Second level Iterator (secondlevel_iter)firstlevel_iter is used to figure out the secondlevel_iter to use, and secondlevel_iter points to the actual data that we are reading.

Example:

RocksDB uses TwoLevelIterator to read SST files, firstlevel_iter is a BlockIter on the SST file Index block and secondlevel_iter is a BlockIter on a Data block.

Let's look at this simplified representation of an SST file, we have 4 Data blocks and 1 Index Block

[Data block, offset: 0x0000]
KEY1  | VALUE1
KEY2  | VALUE2
KEY3  | VALUE3
[Data Block, offset: 0x0100]
KEY4  | VALUE4
KEY7  | VALUE7
[Data Block, offset: 0x0250]
KEY8  | VALUE8
KEY9  | VALUE9
[Data Block, offset: 0x0350]
KEY11 | VALUE11
KEY15 | VALUE15
[Index Block, offset: 0x0500]
KEY3  | 0x0000
KEY7  | 0x0100
KEY9  | 0x0250
KEY15 | 0x0500

To read this file we will create a TwoLevelIterator with

• firstlevel_iter => BlockIter over Index block
• secondlevel_iter => BlockIter over Data block that will be determined by firstlevel_iterWhen we ask our TwoLevelIterator to Seek to KEY8 for example, It will first use firstlevel_iter (BlockIter over Index block) to figure out which block may contain this key. this will lead us to set the secondlevel_iter to be (BlockIter over data block with offset 0x0250). We will then use the secondlevel_iter to find our key & value in the data block.