Trait sled::MergeOperator

pub trait MergeOperator: Fn(&[u8], Option<&[u8]>, &[u8]) -> Option<Vec<u8>> { }

A function that may be configured on a particular shared Tree that will be applied as a kind of read-modify-write operator to any values that are written using the Tree::merge method.

The first argument is the key. The second argument is the optional existing value that was in place before the merged value being applied. The Third argument is the data being merged into the item.

You may return None to delete the value completely.

Merge operators are shared by all instances of a particular Tree. Different merge operators may be set on different Trees.

Examples

use sled::{Config, IVec};

fn concatenate_merge(
  _key: &[u8],               // the key being merged
  old_value: Option<&[u8]>,  // the previous value, if one existed
  merged_bytes: &[u8]        // the new bytes being merged in
) -> Option<Vec<u8>> {       // set the new value, return None to delete
  let mut ret = old_value
    .map(|ov| ov.to_vec())
    .unwrap_or_else(|| vec![]);

  ret.extend_from_slice(merged_bytes);

  Some(ret)
}

let config = Config::new()
  .temporary(true);

let tree = config.open()?;
tree.set_merge_operator(concatenate_merge);

let k = b"k1";

tree.insert(k, vec![0]);
tree.merge(k, vec![1]);
tree.merge(k, vec![2]);
assert_eq!(tree.get(k), Ok(Some(IVec::from(vec![0, 1, 2]))));

// Replace previously merged data. The merge function will not be called.
tree.insert(k, vec![3]);
assert_eq!(tree.get(k), Ok(Some(IVec::from(vec![3]))));

// Merges on non-present values will cause the merge function to be called
// with `old_value == None`. If the merge function returns something (which it
// does, in this case) a new value will be inserted.
tree.remove(k);
tree.merge(k, vec![4]);
assert_eq!(tree.get(k), Ok(Some(IVec::from(vec![4]))));

Implementors

impl<F> MergeOperator for Fwhere F: Fn(&[u8], Option<&[u8]>, &[u8]) -> Option<Vec<u8>>,