BadgerDB is an embeddable, persistent, simple and fast key-value (KV) database written in pure Go. It's meant to be a performant alternative to non-Go-based key-value stores like RocksDB.
Badger is stable and is being used to serve data sets worth hundreds of
terabytes. Badger supports concurrent ACID transactions with serializable
snapshot isolation (SSI) guarantees. A Jepsen-style bank test runs nightly for
8h, with --race
flag and ensures maintainance of transactional guarantees.
Badger has also been tested to work with filesystem level anomalies, to ensure
persistence and consistency.
Badger v1.0 was released in Nov 2017, with a Badger v2.0 release coming up in a few months. The Changelog is kept fairly up-to-date.
- Getting Started
- Resources
- Contact
- Design
- Other Projects Using Badger
- Frequently Asked Questions
To start using Badger, install Go 1.8 or above and run go get
:
$ go get github.com/dgraph-io/badger/...
This will retrieve the library and install the badger_info
command line
utility into your $GOBIN
path.
The top-level object in Badger is a DB
. It represents multiple files on disk
in specific directories, which contain the data for a single database.
To open your database, use the badger.Open()
function, with the appropriate
options. The Dir
and ValueDir
options are mandatory and must be
specified by the client. They can be set to the same value to simplify things.
package main
import (
"log"
"github.com/dgraph-io/badger"
)
func main() {
// Open the Badger database located in the /tmp/badger directory.
// It will be created if it doesn't exist.
opts := badger.DefaultOptions
opts.Dir = "/tmp/badger"
opts.ValueDir = "/tmp/badger"
db, err := badger.Open(opts)
if err != nil {
log.Fatal(err)
}
defer db.Close()
// Your code here…
}
Please note that Badger obtains a lock on the directories so multiple processes cannot open the same database at the same time.
To start a read-only transaction, you can use the DB.View()
method:
err := db.View(func(txn *badger.Txn) error {
// Your code here…
return nil
})
You cannot perform any writes or deletes within this transaction. Badger ensures that you get a consistent view of the database within this closure. Any writes that happen elsewhere after the transaction has started, will not be seen by calls made within the closure.
To start a read-write transaction, you can use the DB.Update()
method:
err := db.Update(func(txn *badger.Txn) error {
// Your code here…
return nil
})
All database operations are allowed inside a read-write transaction.
Always check the returned error value. If you return an error within your closure it will be passed through.
An ErrConflict
error will be reported in case of a conflict. Depending on the state
of your application, you have the option to retry the operation if you receive
this error.
An ErrTxnTooBig
will be reported in case the number of pending writes/deletes in
the transaction exceed a certain limit. In that case, it is best to commit the
transaction and start a new transaction immediately. Here is an example (we are
not checking for errors in some places for simplicity):
updates := make(map[string]string)
txn := db.NewTransaction(true)
for k,v := range updates {
if err := txn.Set([]byte(k),[]byte(v)); err == ErrTxnTooBig {
_ = txn.Commit()
txn = db.NewTransaction(..)
_ = txn.Set([]byte(k),[]byte(v))
}
}
_ = txn.Commit()
The DB.View()
and DB.Update()
methods are wrappers around the
DB.NewTransaction()
and Txn.Commit()
methods (or Txn.Discard()
in case of
read-only transactions). These helper methods will start the transaction,
execute a function, and then safely discard your transaction if an error is
returned. This is the recommended way to use Badger transactions.
However, sometimes you may want to manually create and commit your
transactions. You can use the DB.NewTransaction()
function directly, which
takes in a boolean argument to specify whether a read-write transaction is
required. For read-write transactions, it is necessary to call Txn.Commit()
to ensure the transaction is committed. For read-only transactions, calling
Txn.Discard()
is sufficient. Txn.Commit()
also calls Txn.Discard()
internally to cleanup the transaction, so just calling Txn.Commit()
is
sufficient for read-write transaction. However, if your code doesn’t call
Txn.Commit()
for some reason (for e.g it returns prematurely with an error),
then please make sure you call Txn.Discard()
in a defer
block. Refer to the
code below.
// Start a writable transaction.
txn := db.NewTransaction(true)
defer txn.Discard()
// Use the transaction...
err := txn.Set([]byte("answer"), []byte("42"))
if err != nil {
return err
}
// Commit the transaction and check for error.
if err := txn.Commit(nil); err != nil {
return err
}
The first argument to DB.NewTransaction()
is a boolean stating if the transaction
should be writable.
Badger allows an optional callback to the Txn.Commit()
method. Normally, the
callback can be set to nil
, and the method will return after all the writes
have succeeded. However, if this callback is provided, the Txn.Commit()
method returns as soon as it has checked for any conflicts. The actual writing
to the disk happens asynchronously, and the callback is invoked once the
writing has finished, or an error has occurred. This can improve the throughput
of the application in some cases. But it also means that a transaction is not
durable until the callback has been invoked with a nil
error value.
To save a key/value pair, use the Txn.Set()
method:
err := db.Update(func(txn *badger.Txn) error {
err := txn.Set([]byte("answer"), []byte("42"))
return err
})
This will set the value of the "answer"
key to "42"
. To retrieve this
value, we can use the Txn.Get()
method:
err := db.View(func(txn *badger.Txn) error {
item, err := txn.Get([]byte("answer"))
handle(err)
var valNot, valCopy []byte
err := item.Value(func(val []byte) error {
// This func with val would only be called if item.Value encounters no error.
// Accessing val here is valid.
fmt.Printf("The answer is: %s\n", val)
// Copying or parsing val is valid.
valCopy = append([]byte{}, val...)
// Assigning val slice to another variable is NOT OK.
valNot = val // Do not do this.
return nil
})
handle(err)
// DO NOT access val here. It is the most common cause of bugs.
fmt.Printf("NEVER do this. %s\n", valNot)
// You must copy it to use it outside item.Value(...).
fmt.Printf("The answer is: %s\n", valCopy)
// Alternatively, you could also use item.ValueCopy().
valCopy, err = item.ValueCopy(nil)
handle(err)
fmt.Printf("The answer is: %s\n", valCopy)
return nil
})
Txn.Get()
returns ErrKeyNotFound
if the value is not found.
Please note that values returned from Get()
are only valid while the
transaction is open. If you need to use a value outside of the transaction
then you must use copy()
to copy it to another byte slice.
Use the Txn.Delete()
method to delete a key.
To get unique monotonically increasing integers with strong durability, you can
use the DB.GetSequence
method. This method returns a Sequence
object, which
is thread-safe and can be used concurrently via various goroutines.
Badger would lease a range of integers to hand out from memory, with the
bandwidth provided to DB.GetSequence
. The frequency at which disk writes are
done is determined by this lease bandwidth and the frequency of Next
invocations. Setting a bandwith too low would do more disk writes, setting it
too high would result in wasted integers if Badger is closed or crashes.
To avoid wasted integers, call Release
before closing Badger.
seq, err := db.GetSequence(key, 1000)
defer seq.Release()
for {
num, err := seq.Next()
}
Badger provides support for unordered merge operations. You can define a func
of type MergeFunc
which takes in an existing value, and a value to be
merged with it. It returns a new value which is the result of the merge
operation. All values are specified in byte arrays. For e.g., here is a merge
function (add
) which adds a uint64
value to an existing uint64
value.
func uint64ToBytes(i uint64) []byte {
var buf [8]byte
binary.BigEndian.PutUint64(buf[:], i)
return buf[:]
}
func bytesToUint64(b []byte) uint64 {
return binary.BigEndian.Uint64(b)
}
// Merge function to add two uint64 numbers
func add(existing, new []byte) []byte {
return uint64ToBytes(bytesToUint64(existing) + bytesToUint64(new))
}
This function can then be passed to the DB.GetMergeOperator()
method, along
with a key, and a duration value. The duration specifies how often the merge
function is run on values that have been added using the MergeOperator.Add()
method.
MergeOperator.Get()
method can be used to retrieve the cumulative value of the key
associated with the merge operation.
key := []byte("merge")
m := db.GetMergeOperator(key, add, 200*time.Millisecond)
defer m.Stop()
m.Add(uint64ToBytes(1))
m.Add(uint64ToBytes(2))
m.Add(uint64ToBytes(3))
res, err := m.Get() // res should have value 6 encoded
fmt.Println(bytesToUint64(res))
Badger allows setting an optional Time to Live (TTL) value on keys. Once the TTL has
elapsed, the key will no longer be retrievable and will be eligible for garbage
collection. A TTL can be set as a time.Duration
value using the Txn.SetWithTTL()
API method.
An optional user metadata value can be set on each key. A user metadata value
is represented by a single byte. It can be used to set certain bits along
with the key to aid in interpreting or decoding the key-value pair. User
metadata can be set using the Txn.SetWithMeta()
API method.
Txn.SetEntry()
can be used to set the key, value, user metatadata and TTL,
all at once.
To iterate over keys, we can use an Iterator
, which can be obtained using the
Txn.NewIterator()
method. Iteration happens in byte-wise lexicographical sorting
order.
err := db.View(func(txn *badger.Txn) error {
opts := badger.DefaultIteratorOptions
opts.PrefetchSize = 10
it := txn.NewIterator(opts)
defer it.Close()
for it.Rewind(); it.Valid(); it.Next() {
item := it.Item()
k := item.Key()
err := item.Value(func(v []byte) error {
fmt.Printf("key=%s, value=%s\n", k, v)
return nil
})
if err != nil {
return err
}
}
return nil
})
The iterator allows you to move to a specific point in the list of keys and move forward or backward through the keys one at a time.
By default, Badger prefetches the values of the next 100 items. You can adjust
that with the IteratorOptions.PrefetchSize
field. However, setting it to
a value higher than GOMAXPROCS (which we recommend to be 128 or higher)
shouldn’t give any additional benefits. You can also turn off the fetching of
values altogether. See section below on key-only iteration.
To iterate over a key prefix, you can combine Seek()
and ValidForPrefix()
:
db.View(func(txn *badger.Txn) error {
it := txn.NewIterator(badger.DefaultIteratorOptions)
defer it.Close()
prefix := []byte("1234")
for it.Seek(prefix); it.ValidForPrefix(prefix); it.Next() {
item := it.Item()
k := item.Key()
err := item.Value(func(v []byte) error {
fmt.Printf("key=%s, value=%s\n", k, v)
return nil
})
if err != nil {
return err
}
}
return nil
})
Badger supports a unique mode of iteration called key-only iteration. It is
several order of magnitudes faster than regular iteration, because it involves
access to the LSM-tree only, which is usually resident entirely in RAM. To
enable key-only iteration, you need to set the IteratorOptions.PrefetchValues
field to false
. This can also be used to do sparse reads for selected keys
during an iteration, by calling item.Value()
only when required.
err := db.View(func(txn *badger.Txn) error {
opts := badger.DefaultIteratorOptions
opts.PrefetchValues = false
it := txn.NewIterator(opts)
defer it.Close()
for it.Rewind(); it.Valid(); it.Next() {
item := it.Item()
k := item.Key()
fmt.Printf("key=%s\n", k)
}
return nil
})
Badger values need to be garbage collected, because of two reasons:
-
Badger keeps values separately from the LSM tree. This means that the compaction operations that clean up the LSM tree do not touch the values at all. Values need to be cleaned up separately.
-
Concurrent read/write transactions could leave behind multiple values for a single key, because they are stored with different versions. These could accumulate, and take up unneeded space beyond the time these older versions are needed.
Badger relies on the client to perform garbage collection at a time of their choosing. It provides the following methods, which can be invoked at an appropriate time:
DB.PurgeOlderVersions()
: Is no longer needed since v1.5.0. Badger's LSM tree automatically discards older/invalid versions of keys.DB.RunValueLogGC()
: This method is designed to do garbage collection while Badger is online. Along with randomly picking a file, it uses statistics generated by the LSM-tree compactions to pick files that are likely to lead to maximum space reclamation.
It is recommended that this method be called during periods of low activity in your system, or periodically. One call would only result in removal of at max one log file. As an optimization, you could also immediately re-run it whenever it returns nil error (indicating a successful value log GC).
ticker := time.NewTicker(5 * time.Minute)
defer ticker.Stop()
for range ticker.C {
again:
err := db.RunValueLogGC(0.7)
if err == nil {
goto again
}
}
There are two public API methods DB.Backup()
and DB.Load()
which can be
used to do online backups and restores. Badger v0.9 provides a CLI tool
badger
, which can do offline backup/restore. Make sure you have $GOPATH/bin
in your PATH to use this tool.
The command below will create a version-agnostic backup of the database, to a
file badger.bak
in the current working directory
badger backup --dir <path/to/badgerdb>
To restore badger.bak
in the current working directory to a new database:
badger restore --dir <path/to/badgerdb>
See badger --help
for more details.
If you have a Badger database that was created using v0.8 (or below), you can
use the badger_backup
tool provided in v0.8.1, and then restore it using the
command above to upgrade your database to work with the latest version.
badger_backup --dir <path/to/badgerdb> --backup-file badger.bak
Badger's memory usage can be managed by tweaking several options available in
the Options
struct that is passed in when opening the database using
DB.Open
.
Options.ValueLogLoadingMode
can be set tooptions.FileIO
(instead of the defaultoptions.MemoryMap
) to avoid memory-mapping log files. This can be useful in environments with low RAM.- Number of memtables (
Options.NumMemtables
)- If you modify
Options.NumMemtables
, also adjustOptions.NumLevelZeroTables
andOptions.NumLevelZeroTablesStall
accordingly.
- If you modify
- Number of concurrent compactions (
Options.NumCompactors
) - Mode in which LSM tree is loaded (
Options.TableLoadingMode
) - Size of table (
Options.MaxTableSize
) - Size of value log file (
Options.ValueLogFileSize
)
If you want to decrease the memory usage of Badger instance, tweak these options (ideally one at a time) until you achieve the desired memory usage.
Badger records metrics using the expvar package, which is included in the Go standard library. All the metrics are documented in y/metrics.go file.
expvar
package adds a handler in to the default HTTP server (which has to be
started explicitly), and serves up the metrics at the /debug/vars
endpoint.
These metrics can then be collected by a system like Prometheus, to get
better visibility into what Badger is doing.
- Introducing Badger: A fast key-value store written natively in Go
- Make Badger crash resilient with ALICE
- Badger vs LMDB vs BoltDB: Benchmarking key-value databases in Go
- Concurrent ACID Transactions in Badger
Badger was written with these design goals in mind:
- Write a key-value database in pure Go.
- Use latest research to build the fastest KV database for data sets spanning terabytes.
- Optimize for SSDs.
Badger’s design is based on a paper titled WiscKey: Separating Keys from Values in SSD-conscious Storage.
Feature | Badger | RocksDB | BoltDB |
---|---|---|---|
Design | LSM tree with value log | LSM tree only | B+ tree |
High Read throughput | Yes | No | Yes |
High Write throughput | Yes | Yes | No |
Designed for SSDs | Yes (with latest research 1) | Not specifically 2 | No |
Embeddable | Yes | Yes | Yes |
Sorted KV access | Yes | Yes | Yes |
Pure Go (no Cgo) | Yes | No | Yes |
Transactions | Yes, ACID, concurrent with SSI3 | Yes (but non-ACID) | Yes, ACID |
Snapshots | Yes | Yes | Yes |
TTL support | Yes | Yes | No |
1 The WISCKEY paper (on which Badger is based) saw big wins with separating values from keys, significantly reducing the write amplification compared to a typical LSM tree.
2 RocksDB is an SSD optimized version of LevelDB, which was designed specifically for rotating disks. As such RocksDB's design isn't aimed at SSDs.
3 SSI: Serializable Snapshot Isolation. For more details, see the blog post Concurrent ACID Transactions in Badger
We have run comprehensive benchmarks against RocksDB, Bolt and LMDB. The benchmarking code, and the detailed logs for the benchmarks can be found in the badger-bench repo. More explanation, including graphs can be found the blog posts (linked above).
Below is a list of known projects that use Badger:
- 0-stor - Single device object store.
- Dgraph - Distributed graph database.
- Dispatch Protocol - Blockchain protocol for distributed application data analytics.
- Sandglass - distributed, horizontally scalable, persistent, time sorted message queue.
- Usenet Express - Serving over 300TB of data with Badger.
- go-ipfs - Go client for the InterPlanetary File System (IPFS), a new hypermedia distribution protocol.
- gorush - A push notification server written in Go.
- emitter - Scalable, low latency, distributed pub/sub broker with message storage, uses MQTT, gossip and badger.
- GarageMQ - AMQP server written in Go.
- RedixDB - A real-time persistent key-value store with the same redis protocol.
If you are using Badger in a project please send a pull request to add it to the list.
- My writes are getting stuck. Why?
Update: With the new Value(func(v []byte))
API, this deadlock can no longer
happen.
The following is true for users on Badger v1.x.
This can happen if a long running iteration with Prefetch
is set to false, but
a Item::Value
call is made internally in the loop. That causes Badger to
acquire read locks over the value log files to avoid value log GC removing the
file from underneath. As a side effect, this also blocks a new value log GC
file from being created, when the value log file boundary is hit.
Please see Github issues #293 and #315.
There are multiple workarounds during iteration:
- Use
Item::ValueCopy
instead ofItem::Value
when retrieving value. - Set
Prefetch
to true. Badger would then copy over the value and release the file lock immediately. - When
Prefetch
is false, don't callItem::Value
and do a pure key-only iteration. This might be useful if you just want to delete a lot of keys. - Do the writes in a separate transaction after the reads.
- My writes are really slow. Why?
Are you creating a new transaction for every single key update, and waiting for
it to Commit
fully before creating a new one? This will lead to very low
throughput.
We have created WriteBatch
API which provides a way to batch up
many updates into a single transaction and Commit
that transaction using
callbacks to avoid blocking. This amortizes the cost of a transaction really
well, and provides the most efficient way to do bulk writes.
wb := db.NewWriteBatch()
defer wb.Cancel()
for i := 0; i < N; i++ {
err := wb.Set(key(i), value(i), 0) // Will create txns as needed.
handle(err)
}
handle(wb.Flush()) // Wait for all txns to finish.
Note that WriteBatch
API does not allow any reads. For read-modify-write
workloads, you should be using the Transaction
API.
- I don't see any disk write. Why?
If you're using Badger with SyncWrites=false
, then your writes might not be written to value log
and won't get synced to disk immediately. Writes to LSM tree are done inmemory first, before they
get compacted to disk. The compaction would only happen once MaxTableSize
has been reached. So, if
you're doing a few writes and then checking, you might not see anything on disk. Once you Close
the database, you'll see these writes on disk.
- Reverse iteration doesn't give me the right results.
Just like forward iteration goes to the first key which is equal or greater than the SEEK key, reverse iteration goes to the first key which is equal or lesser than the SEEK key. Therefore, SEEK key would not be part of the results. You can typically add a 0xff
byte as a suffix to the SEEK key to include it in the results. See the following issues: #436 and #347.
- Which instances should I use for Badger?
We recommend using instances which provide local SSD storage, without any limit on the maximum IOPS. In AWS, these are storage optimized instances like i3. They provide local SSDs which clock 100K IOPS over 4KB blocks easily.
- I'm getting a closed channel error. Why?
panic: close of closed channel
panic: send on closed channel
If you're seeing panics like above, this would be because you're operating on a closed DB. This can happen, if you call Close()
before sending a write, or multiple times. You should ensure that you only call Close()
once, and all your read/write operations finish before closing.
- Are there any Go specific settings that I should use?
We highly recommend setting a high number for GOMAXPROCS, which allows Go to observe the full IOPS throughput provided by modern SSDs. In Dgraph, we have set it to 128. For more details, see this thread.
- Are there any linux specific settings that I should use?
We recommend setting max file descriptors to a high number depending upon the expected size of you data.
- Please use discuss.dgraph.io for questions, feature requests and discussions.
- Please use Github issue tracker for filing bugs or feature requests.
- Join
.
- Follow us on Twitter @dgraphlabs.