Engineering glossary
Short definitions with concrete examples. In an article, tap a term with a dotted underline to read its definition in place.
- B-tree
A balanced search tree that keeps keys ordered and stores many keys in each node. Database indexes commonly use B-tree variants to find a starting point and scan a range efficiently.
An index on created_at can seek to a timestamp, then read neighboring entries.
Read the explanation- Boxing
Converting a value type into an object or interface reference by copying its value into a boxed object. Later changes to the original variable do not update that copy.
After int n = 42; object box = n; n = 7;, the box still contains 42.
Read the explanation- Composite cursor
A pagination cursor containing multiple ordering values, including a unique tiebreaker, so the next query can resume after one unambiguous position.
Use both created_at and id when several posts share a timestamp.
Read the explanation- Congestion window
A sender-controlled limit on data in flight, adjusted in response to network conditions. The receiver’s advertised window imposes a separate flow-control limit.
With a congestion window of 8 packets and a receive window of 5, at most 5 packets may be in flight in the simplified model.
Read the explanation- Covering index
An index that contains all the columns a particular query needs. Whether it avoids visiting the base table also depends on the database and its visibility rules.
In SQL Server, an index on Email that includes Name can cover a query selecting Name by Email.
Read the explanation- Head-of-line blocking
Later work waits behind an earlier missing or unfinished item. In a reliable ordered stream, later data can arrive but cannot be delivered until the gap is filled.
Packets 11 and 12 wait in the reorder buffer while packet 10 is retransmitted.
Read the explanation- Location rule
An Nginx configuration block selected by matching a normalized request path against exact, prefix, or regular expression patterns. Query parameters do not participate in this match.
location = /health also matches a request for /health?check=ready.
Read the explanation- Middleware
A component in an HTTP request pipeline that can run work before and after the next component, or return a response without invoking it.
Logging middleware can start a timer, await the next component, and then record the elapsed time.
Read the explanation- MVCC
Multiversion concurrency control: a database keeps row versions and uses visibility rules to decide which versions a transaction can read. A pagination cursor alone does not preserve a database snapshot.
Two page requests in separate transactions can observe different committed data.
Read the explanation- Time quantum
The maximum uninterrupted CPU time a runnable process gets in one turn under a scheduling policy such as Round Robin. A process can finish or block before using the entire quantum.
A process needing 2 ms finishes after 2 ms even when the quantum is 4 ms.
Read the explanation