- System architectures
The system architecture of a distributed system is how the system’s software and hardware are organized.
There is a distinction between the logical organization of distributed systems (software architecture) and the physical organization [1, Pp. 55-6].
A component is a modular unit with a well-defined interface. Components should be replaceable without requiring the system to be powered down [1, P. 56].
A connector is a mechanism that handles communication, coordination, or cooperation between components [1, P. 56].
This section documents some common system architectures.
In client-server architectures, processes are divided into two (sometimes overlapping) groups: servers and clients [1, P. 76].
A server is a process that implements a specific service.
A client is a process that uses a server’s service by sending it a request and waiting for a response [1, P. 76].
A client can communicate with a server using a connectionless protocol, like UDP. The downside of using a connectionless protocol is that handling transmission errors is difficult [1, P. 77].
One solution is to use a connection-based protocol, like TCP.
The distinction between client and server is often not clear in the client-server model [1, P. 77].
The client-server model is a two-tiered architecture [1, P. 78].
Many distributed applications are split into three layers:
- User interface layer
- Processing layer
- Data layer
These layers can be distributed across different machines. An example would be a website where the browser operates at the user interface layer, a server is the processing layer, and a database operates at the data layer [1, Pp. 78,80].
The server running at the processing layer will act as a client to the database at the data layer [1, P. 80].
Peer-to-peer systems support distribution across clients and servers. A client or server might be physically split into logically equivalent parts, where each part is operating on its own share of the complete data set [1, P. 81].
In a peer-to-peer system, all processes that constitute the system are equal. Interaction between processes is symmetric: a process will act as a client and as a server at the same time [1, P. 81].
Peer-to-peer architectures are concerned with organizing processes in an overlay network. An overlay network is a network that is formed of the processes and the links that represent possible communication channels (usually TCP). An overlay network can either be structured or unstructured [1, P. 81].
In a structured peer-to-peer system, the overlay follows a predetermined topology (e.g. a ring or a grid). The topology is used to look up data.
Generally, structured peer-to-peer systems use a semantic-free index to access data. Each data item maintained by the system is uniquely associated with a key, and the key is used as an index:
The peer-to-peer system is then responsible for storing key-value pairs. In this case, a node is assigned an identifier from all hash values, and each node is responsible for storing data associated with a subset of keys. This architecture is known as a DHT (distributed hash table) [1, P. 82]. A DHT makes it easy for any node to find the correct node for a key:
In an unstructured peer-to-peer system, each node maintains a dynamic list of neighbors. Commonly, when a node joins an unstructured peer-to-peer system it contacts a well-known node in order to obtain a list of peers [1, P. 84].
Unstructured peer-to-peer systems must search for a requested value. Some examples of search methods are:
- Random walk
In flooding, an issuing node, , passes a request for an item to each of its neighbors. A request is ignored if the receiving node, , has seen it before. If the request hasn’t been seen before, will search locally for the item. If the item isn’t available locally, will issue a request to each of its neighbors and return the response to if it receives one [1, P. 85].
Random walk involves an issuing node, , asking one of its neighbors at random. If the neighbor cannot satisfy the request, asks another neighbor at random, and so on until it receives an answer [1, P. 85].
Locating data items can become difficult as the network grows, since there is no deterministic way to route a lookup request to a specific node. Some peer-to-peer systems use special index nodes to overcome this problem [1, P. 87].
An alternative is to abandon symmetric communication and use a broker node to coordinate communication between a client and a server.
Nodes that maintain an index or act as a broker are known as super peers. Super peers are often organized in a peer-to-peer relationship.
A pubsub system is based on asynchronous communication between publishers and subscribers.
Publishers publish events and relevant subscribers are notified of the published event.
There are two common forms of pubsub systems:
- Topic-based systems
- Content-based systems
In a topic-based pubsub system, a subscriber subscribes to a named logical channel (a topic). Events published to the channel typically contain <attribute, value> pairs.
In a content-based publish-subscribe system, a subscription can also consist of <attribute, value> pairs. An event is only sent to a subscriber if the event’s attributes match constraints provided by the subscriber [1, P. 70].
This section documents common components used in distributed systems.
A load balancer is a server that distributes traffic between a cluster of backend servers. Load balancers are used to implement horizontal scaling.
A load balancer can be implemented in hardware (expensive) or as software running on a commodity machine (cheaper).
Load balancers periodically perform health checks to determine which nodes in a cluster are available. They distribute requests between the healthy nodes (commonly requests are distributed round-robin style) [2, P. 8].
The two main types of load balancer are application load balancers and network load balancers.
An application load balancer (also known as a layer 7 load balancer) can use application layer information to determine how to route requests. For example, they can perform HTTP path-based routing.
A network load balancer (also known as a layer 4 load balancer) performs NAT to forward transport packets using the IP address and port number to make load balancing decisions.
To avoid a load balancer becoming a single point of failure you can have two servers running load balancers in an active-passive configuration using a single virtual IP address. If the active server goes down, the passive server will take over and start receiving traffic for the virtual IP.
A reverse proxy is a server that receives incoming requests and forwards them to the relevant server.
The benefits of using a reverse proxy include:
- Load balancing
- Controlling incoming traffic to a private network
- SSL encryption/decryption
An API gateway is a special type of reverse proxy that takes API calls from a client and routes them to the correct backend service. API gateways often invoke multiple backend services and then aggregate the result.
-  A. Tanenbaum and M. van Steen, Distributed Systems, 3.01 ed. Pearson Education, Inc., 2017.
-  K. Salchow, “Load balancing 101: Nuts and bolts,” White Paper, F5 Networks, Inc, 2007.