Add OptionsUtil::LoadOptionsFromFile() API

Summary:
This patch adds OptionsUtil::LoadOptionsFromFile() and
OptionsUtil::LoadLatestOptionsFromDB(), which allow developers
to construct DBOptions and ColumnFamilyOptions from a RocksDB
options file.  Note that most pointer-typed options such as
merge_operator will not be constructed.

With this API, developers no longer need to remember all the
options in order to reopen an existing rocksdb instance like
the following:

  DBOptions db_options;
  std::vector<std::string> cf_names;
  std::vector<ColumnFamilyOptions> cf_opts;

  // Load primitive-typed options from an existing DB
  OptionsUtil::LoadLatestOptionsFromDB(
      dbname, &db_options, &cf_names, &cf_opts);

  // Initialize necessary pointer-typed options
  cf_opts[0].merge_operator.reset(new MyMergeOperator());
  ...

  // Construct the vector of ColumnFamilyDescriptor
  std::vector<ColumnFamilyDescriptor> cf_descs;
  for (size_t i = 0; i < cf_opts.size(); ++i) {
    cf_descs.emplace_back(cf_names[i], cf_opts[i]);
  }

  // Open the DB
  DB* db = nullptr;
  std::vector<ColumnFamilyHandle*> cf_handles;
  auto s = DB::Open(db_options, dbname, cf_descs,
                    &handles, &db);

Test Plan:
Augment existing tests in column_family_test
options_test
db_test

Reviewers: igor, IslamAbdelRahman, sdong, anthony

Reviewed By: anthony

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D49095

1 files changed, 1 insertions, 0 deletions

diff --git a/HISTORY.md b/HISTORY.md
index a54c13da4..a6a4018db 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -6,6 +6,7 @@
 * Add GetAggregatedIntProperty(), which returns the sum of the GetIntProperty of all the column families. 
 * Add MemoryUtil in rocksdb/utilities/memory.h.  It currently offers a way to get the memory usage by type from a list rocksdb instances.
 * RocksDB will now persist options under the same directory as the RocksDB database on successful DB::Open, CreateColumnFamily, DropColumnFamily, and SetOptions.
+* Introduce LoadLatestOptions() in rocksdb/utilities/options_util.h.  This function can construct the latest DBOptions / ColumnFamilyOptions used by the specified RocksDB intance.
 ### Public API Changes
 * CompactionFilter::Context includes information of Column Family ID
 * The need-compaction hint given by TablePropertiesCollector::NeedCompact() will be persistent and recoverable after DB recovery. This introduces a breaking format change. If you use this experimental feature, including NewCompactOnDeletionCollectorFactory() in the new version, you may not be able to directly downgrade the DB back to version 4.0 or lower.