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SSTables 3.0 Statistics File Format

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Michael Livshin edited this page Feb 20, 2022 · 12 revisions

This file stores metadata for SSTable. There are 4 types of metadata:

  1. Validation metadata - used to validate SSTable correctness
  2. Compaction metadata - used for compaction
  3. Statistics - some information about SSTable which is loaded into memory and used for faster reads/compactions
  4. Serialization header - keeps information about SSTable schema

General structure

The file is composed of two parts. First part is a table of content which allows quick access to a selected metadata. Second part is a sequence of metadata stored one after the other. Let's define array template that will be used in this document.

struct array<LengthType, ElementType> {
  LengthType number_of_elements;
  ElementType elements[number_of_elements];
}

Table of content

using toc = array<be32<int32_t>, toc_entry>;

struct toc_entry {
  // Type of metadata
  // | Type                 | Integer representation |
  // |----------------------|------------------------|
  // | Validation metadata  | 0                      |
  // | Compaction metadata  | 1                      |
  // | Statistics           | 2                      |
  // | Serialization header | 3                      |
  be32<int32_t> type;
  // Offset, in the file, at which this metadata entry starts
  be32<int32_t> offset;
}

The toc array is sorted by the type field of its members.

Validation metadata entry

struct validation_metadata {
  // Name of partitioner used to create this SSTable.
  // Represented by UTF8 string encoded using modified UTF-8 encoding.
  // You can read more about this encoding in:
  // https://docs.oracle.com/javase/7/docs/api/java/io/DataInput.html#modified-utf-8
  // https://docs.oracle.com/javase/7/docs/api/java/io/DataInput.html#readUTF()
  Modified_UTF-8_String partitioner_name;
  // The probability of false positive matches in the bloom filter for this SSTable
  be64<double> bloom_filter_fp_chance;
}

Compaction metadata entry

// Serialized HyperLogLogPlus which can be used to estimate the number of partition keys in the SSTable.
// If this is not present then the same estimation can be computed using Summary file.
// Encoding is described in:
// https://github.com/addthis/stream-lib/blob/master/src/main/java/com/clearspring/analytics/stream/cardinality/HyperLogLogPlus.java
using compaction_metadata = array<be32<int32_t>, be8>;

Statistics entry

This entry contains some parts of EstimatedHistogram, StreamingHistogram and CommitLogPosition types. Let's have a look at them first.

EstimatedHistogram

// Each bucket represents values from (previous bucket offset, current offset].
// Offset for last bucket is +inf.
using estimated_histogram = array<be32<int32_t>, bucket>;

struct bucket {
  // Offset of the previous bucket
  // In the first bucket this is offset of the first bucket itself because there's no previous bucket.
  // The offset of the first bucket is repeated in second bucket as well.
  be64<int64_t> prev_bucket_offset;
  // This bucket value
  be64<int64_t> value;
}

StreamingHistogram

struct streaming_histogram {
  // Maximum number of buckets this historgam can have
  be32<int32_t> bucket_number_limit;
  array<be32<int32_t>, bucket> buckets;
}

struct bucket {
  // Offset of this bucket
  be64<double> offset;
  // Bucket value
  be64<int64_t> value;
}

CommitLogPosition

struct commit_log_position {
  be64<int64_t> segment_id;
  be32<int32_t> position_in_segment;
}

Whole entry

struct statistics {
  // In bytes, uncompressed sizes of partitions
  estimated_histogram partition_sizes;
  // Number of cells per partition
  estimated_histogram column_counts;
  commit_log_position commit_log_upper_bound;
  // Typically in microseconds since the unix epoch, although this is not enforced
  be64<int64_t> min_timestamp;
  // Typically in microseconds since the unix epoch, although this is not enforced
  be64<int64_t> max_timestamp;
  // In seconds since the unix epoch
  be32<int32_t> min_local_deletion_time;
  // In seconds since the unix epoch
  be32<int32_t> max_local_deletion_time;
  be32<int32_t> min_ttl;
  be32<int32_t> max_ttl;
  // compressed_size / uncompressed_size
  be64<double> compression_rate;
  // Histogram of cell tombstones.
  // Keys are local deletion times of tombstones
  streaming_histogram tombstones;
  be32<int32_t> level;
  // The difference, measured in milliseconds, between repair time and midnight, January 1, 1970 UTC
  be64<int64_t> repaired_at;
  clustering_bound min_clustering_key;
  clustering_bound max_clustering_key;
  be8<bool> has_legacy_counters;
  be64<int64_t> number_of_columns;
  be64<int64_t> number_of_rows;

  // Version MA of SSTable 3.x format ends here.
  // It contains only one commit log position interval - [NONE = new CommitLogPosition(-1, 0), upper bound of commit log]

  commit_log_position commit_log_lower_bound;

  // Version MB of SSTable 3.x format ends here.
  // It contains only one commit log position interval - [lower bound of commit log, upper bound of commit log].

  array<be32<int32_t>, commit_log_interval> commit_log_intervals;

  // Versions MC and MD of SSTable 3.x format end here.

  optional<UUID> originating_host_id; // 3_11_11 and later (ME format)
}

using clustering_bound = array<be32<int32_t>, clustering_column>;
using clustering_column = array<be16<uint16_t>, be8>;

struct commit_log_interval {
  commit_log_position start;
  commit_log_position end;
}

Serialization header

struct serialization_header {
  vint<uint64_t> min_timestamp;
  vint<uint32_t> min_local_deletion_time;
  vint<uint32_t> min_ttl;
  // If partition key has one column then this is the type of this column.
  // Otherwise, this is a CompositeType that contains types of all partition key columns.
  type partition_key_type;
  array<vint<uint32_t>, type> clustering_key_types;
  columns static_columns;
  columns regular_columns;
}

using columns = array<vint<uint32_t>, column>;

struct column {
  array<vint<uint32_t>, be8> name;
  type column_type;
}

// UTF-8 string
using type = array<vint<uint_32_t>, be8>;

Type encoding

Type is just a byte buffer with an unsigned variant integer (32-bit) length. It is a UTF-8 string. All leading spaces, tabs and newlines are skipped. Null or empty string is a bytes type. First segment of non-blank characters should contain only alphanumerical characters and special chars like '-', '+', '.', '_', '&'. This is the name of the type. If type name does not contain any '.' then it gets "org.apache.cassandra.db.marshal." prepended to itself. Then an "instance" static field is taken from this class. If the first non-blank character that follows type name is '(' then "getInstance" static method is invoked instead. Remaining string is passed to this method as a parameter. There are following types:

Type Parametrized
Ascii Type No
Boolean Type No
Bytes Type No
Byte Type No
ColumnToCollection Type Yes
Composite Type Yes
CounterColumn Type No
Date Type No
Decimal Type No
Double Type No
Duration Type No
DynamicComposite Type Yes
Empty Type No
Float Type No
Frozen Type Yes
InetAddress Type No
Int32 Type No
Integer Type No
LexicalUUID Type No
List Type Yes
Long Type No
Map Type Yes
PartitionerDefinedOrder Yes
Reversed Type Yes
Set Type Yes
Short Type No
SimpleDate Type No
Timestamp Type No
Time Type No
TimeUUID Type No
Tuple Type Yes
User Type Yes
UTF8 Type No
UUID Type No
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