What does creating a connection between an application and database mean? - database

Question

When we say we have created a connection between a database and an application (that can be stored in connection pool), what really a "connection" means here?
Does it got anything to do with establishing a TCP/ TLS connection?
Does it load the database schema with every connection?
What happens to a connection (that are already loaded in application connection pool) when the database schema changes, and there is an active transaction going on?

Answer 1

"A Connection" is nothing but details of a Socket, with extra details (like username, password, etc.). Each connection have a different socket connection.
For example:
Connection 1:
Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]
Connection 2:
Socket[addr=localhost/127.0.0.1,port=1030,localport=51246]
I have created two connection in a single JVM process to demonstrate how the server knows in which Socket the reply is to be sent. A socket, if I define in terms of UNIX is a special file that is used for inter-process communication:
srwxr-xr-x. 1 root root 0 Mar 3 19:30 /tmp/somesocket
When a socket is created (i.e., when this special socket file is created; how to create a socket? and this) operating system creates a file descriptor that points to that file. Server distinguishes the Socket with following attributes:
Ref.
{SRC-IP, SRC-PORT, DEST-IP, DEST-PORT, PROTOCOL}
PROTOCOL: I have used postgres as an example, the socket connection in postgres driver is done with SocksSocketImpl which is a TCP socket implementation (RFC 1928)
Coming back the two connections I have created, if you look closely the localport for both the connections are different, so the server clearly understands where it has to send the reply back.
Now there are limitations on the number of files (or file descriptors) that you can open in an operating system, thus it's recommended not to keep your connections dangling (called connection leak)
Does it load the database schema with every connection?
Answer: No, it's the ResultSet that takes care of it.
What happens to a connection when the database schema changes
Answer: Connection and database schema are two different things. Connection just defines how to communicate with another process. Database schema is a contract between application and database, an application might throw errors that the contract is broken, or it may simply ignore it.
If you are interested in digging more, you should add a breakpoint to a connection object and below is how it looks like (see FileDescriptor)
connection = {Jdbc4Connection#777}
args = {String[0]#776}
connection = {Jdbc4Connection#777}
_clientInfo = null
rsHoldability = 2
savepointId = 0
logger = {Logger#778}
creatingURL = "dbc:postgresql://localhost:1030/postgres"
value = {char[40]#795}
hash = 0
openStackTrace = null
protoConnection = {ProtocolConnectionImpl#780}
serverVersion = "10.7"
cancelPid = 19672
cancelKey = 1633313435
standardConformingStrings = true
transactionState = 0
warnings = null
closed = false
notifications = {ArrayList#796} size = 0
pgStream = {PGStream#797}
host = "localhost"
port = 1030
_int4buf = {byte[4]#802}
_int2buf = {byte[2]#803}
connection = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
created = true
bound = true
connected = true
closed = false
closeLock = {Object#811}
shutIn = false
shutOut = false
impl = {SocksSocketImpl#812} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
server = null
serverPort = 1080
external_address = null
useV4 = false
cmdsock = null
cmdIn = null
cmdOut = null
applicationSetProxy = false
impl = {DualStackPlainSocketImpl#814} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
exclusiveBind = true
isReuseAddress = false
timeout = 0
trafficClass = 0
shut_rd = false
shut_wr = false
socketInputStream = {SocketInputStream#819}
eof = false
impl = {DualStackPlainSocketImpl#814} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
temp = null
socket = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
created = true
bound = true
connected = true
closed = false
closeLock = {Object#811}
shutIn = false
shutOut = false
impl = {SocksSocketImpl#812} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
server = null
serverPort = 1080
external_address = null
useV4 = false
cmdsock = null
cmdIn = null
cmdOut = null
applicationSetProxy = false
impl = {DualStackPlainSocketImpl#814} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
timeout = 0
trafficClass = 0
shut_rd = false
shut_wr = false
socketInputStream = null
socketOutputStream = null
fdUseCount = 0
fdLock = {Object#815}
closePending = false
CONNECTION_NOT_RESET = 0
CONNECTION_RESET_PENDING = 1
CONNECTION_RESET = 2
resetState = 0
resetLock = {Object#816}
stream = false
socket = null
serverSocket = null
fd = {FileDescriptor#817}
address = null
port = 0
localport = 0
oldImpl = false
closing = false
fd = {FileDescriptor#817}
fd = 1260
handle = -1
parent = {SocketInputStream#819}
eof = false
impl = {DualStackPlainSocketImpl#814} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
temp = null
socket = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
closing = false
fd = {FileDescriptor#817}
fd = 1260
handle = -1
parent = {SocketInputStream#819}
eof = false
impl = {DualStackPlainSocketImpl#814} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
exclusiveBind = true
isReuseAddress = false
timeout = 0
trafficClass = 0
shut_rd = false
shut_wr = false
socketInputStream = {SocketInputStream#819}
socketOutputStream = {SocketOutputStream#820}
fdUseCount = 0
fdLock = {Object#821}
closePending = false
CONNECTION_NOT_RESET = 0
CONNECTION_RESET_PENDING = 1
CONNECTION_RESET = 2
resetState = 0
resetLock = {Object#822}
stream = true
socket = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
serverSocket = null
fd = {FileDescriptor#817}
address = {Inet4Address#823} "localhost/127.0.0.1"
port = 1030
localport = 51099
temp = null
socket = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
closing = false
fd = {FileDescriptor#817}
path = null
channel = null
closeLock = {Object#826}
closed = false
otherParents = {ArrayList#833} size = 2
closed = false
path = null
channel = null
closeLock = {Object#826}
closed = false
otherParents = {ArrayList#833} size = 2
closed = false
path = null
channel = null
closeLock = {Object#826}
closed = false
socketOutputStream = {SocketOutputStream#820}
impl = {DualStackPlainSocketImpl#814} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
temp = {byte[1]#843}
socket = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
closing = false
fd = {FileDescriptor#817}
append = false
channel = null
path = null
closeLock = {Object#844}
closed = false
fdUseCount = 0
fdLock = {Object#821}
closePending = false
CONNECTION_NOT_RESET = 0
CONNECTION_RESET_PENDING = 1
CONNECTION_RESET = 2
resetState = 0
resetLock = {Object#822}
stream = true
socket = {Socket#804} "Socket[addr=localhost/127.0.0.1,port=1030,localport=51099]"
serverSocket = null
fd = {FileDescriptor#817}
address = {Inet4Address#823} "localhost/127.0.0.1"
port = 1030
localport = 51099
timeout = 0
trafficClass = 0
shut_rd = false
shut_wr = false
socketInputStream = null
socketOutputStream = null
fdUseCount = 0
fdLock = {Object#815}
closePending = false
CONNECTION_NOT_RESET = 0
CONNECTION_RESET_PENDING = 1
CONNECTION_RESET = 2
resetState = 0
resetLock = {Object#816}
stream = false
socket = null
serverSocket = null
fd = {FileDescriptor#817}
address = null
port = 0
localport = 0
oldImpl = false
pg_input = {VisibleBufferedInputStream#805}
pg_output = {BufferedOutputStream#806}
streamBuffer = null
encoding = {Encoding#807} "UTF-8"
encodingWriter = {OutputStreamWriter#808}
user = "postgres"
database = "postgres"
executor = {QueryExecutorImpl#800}
logger = {Logger#778}
compatible = "9.0"
dbVersionNumber = "10.7"
commitQuery = {SimpleQuery#783} "COMMIT"
rollbackQuery = {SimpleQuery#784} "ROLLBACK"
_typeCache = {TypeInfoCache#785}
prepareThreshold = 5
autoCommit = true
readOnly = false
bindStringAsVarchar = true
firstWarning = null
timestampUtils = {TimestampUtils#786}
typemap = null
fastpath = null
largeobject = null
metadata = null
copyManager = null

Answer 2

Here the connection you are talking about means the opening function that the application invokes to open and read/modify/delete the database or childs of it.
For example if we talk about a PHP file (used to load websites requests in server, like HTML) or a HTML file, where you login in a page that has as name: https://example.com/login.php (PHP) or https://example.com/login.html (HTML) and the page requires to access the adatabase of the users to check if the credentials you insert are correct, if the values given (for e.g: username:"demoUser" and password:"password*1234"), exists in the databse as rows in a specific table. The database can contain infinite tables and infinite rows inside. An example of a simple database with only one table called Users:
username | password | date_created // Table columns
"demoUser" | "password" | "23-03-2019" // Example showed above
"user1213" | "passw0rd" | "04-02-2019" //Second user example
then here above if the application need to verify if the value exist in this database, the operating system of the application will access the database with a simple file reading file is normally with .db and then it will read each rows to find the values.
To do this the code in the login.php/login.html pages invokes the server that runs the file and the server open the database and then the server take the query(what the code request to check in the database), and execute it as if the database was a simple file with (for e.g:) .db. The connection here stands as the query the

Answer 3

To put it in simple words. A "Database connection" is a link between your application process and Database's serving process.
Client side:
When you create a connection your application stores information like: what database address is, what socket is used for the connection, what server process is responsible for processing your requests and etc. This information depends on a connection driver implementation and differs from database to database.
Server side:
When a request from a client application arrives, a database performs authentication and authorization of the client and creates a new process or a thread which is responsible for serving it. Implementation and data loaded by this server process is also vendor-dependent and differs from database to database.
This process of 'preparing' a database for serving a new client takes a good amount of time, and that's where connection pools come to help.
Connection pool:
Connection pool is basically used to reduce the need for openning new connections and wasting time on authentication, authorization, creating server process etc. It allows reusing already established connections.
What happens to a connection (that are already loaded in application connection pool) when the database schema changes, and there is an active transaction going on?
First of all, a database does not know about any connection pools. For the databse it's a client-side feature. What happens also depends on a particular database and its implementation. Usually databases have a blocking mechanism to prevent objects from modifying while they are still in use and vice-versa.