Load balancing algorithms commonly used in Distributed Systems

1. Round Robin

Idea: the requests are distributed to the back-end servers in turn. It treats each back-end server in a balanced way, and does not care about the actual number of connections and the load of the current system;
Code implementation:

private List<String> list = new CopyOnWriteArrayList(); private volatile Integer pos = 0; { list.add("127.0.0.1"); list.add("127.0.0.2"); list.add("127.0.0.3"); } public String getServer() { if (null == list || list.size() <= 0) { return null; } String server = null; synchronized (pos) { if (pos >= list.size()) { pos = 0; } server = list.get(pos++); } return server; }

Summary:

This algorithm is simple, forwarding in turn, and the number of requests per server is average;
Disadvantages: when the performance of servers in a cluster is different, resources will be wasted if they cannot be treated differently;

2. Random method

Idea: through the system random function, according to the size value of the back-end server list, one of them is randomly selected for access; with the increase of the amount of call, the effect is close to the polling algorithm;
Code implementation:

private List<String> list = new CopyOnWriteArrayList(); { list.add("127.0.0.1"); list.add("127.0.0.2"); list.add("127.0.0.3"); list.add("127.0.0.4"); } public String getServerRandom() { if (null == list || list.size() <= 0) { return null; } Random random = new Random(); String server = list.get(random.nextInt(list.size())); return server; }

Summary:

Although the algorithm is simple, it can guarantee the balance in the case of large requests

3. Source address Hash

Idea: the idea of source address hash is to obtain the ip address accessed by the client, get a value through hash function calculation, and use the value to carry out modular operation from the server list; when the server list is unchanged, the same ip is always requested to the same server;
Code implementation:

private List<String> list = new CopyOnWriteArrayList(); { list.add("127.0.0.1"); list.add("127.0.0.2"); list.add("127.0.0.3"); list.add("127.0.0.4"); } public String getServerHash(String hostIp) { if (null == list || list.size() <= 0 || null == hostIp) { return null; } int code = hostIp.hashCode(); int serverPos = list.size() % code; return list.get(serverPos); }

4. Weight Round Robin

Idea: compared with polling algorithm, it adds weight, and the service with super high weight receives more requests;
Code implementation:

private ConcurrentMap<String, Integer> hosts = new ConcurrentHashMap<>(); private volatile Integer pos = 0; { hosts.put("127.0.0.1", 1); hosts.put("127.0.0.2", 2); hosts.put("127.0.0.3", 2); hosts.put("127.0.0.4", 1); } public String getServerRoundRobin() { List<String> list = new ArrayList<>(); for (Map.Entry<String, Integer> entry : hosts.entrySet()) { Integer value = entry.getValue(); for (int i = 0; i < value; i++) { list.add(entry.getKey()); } } String server = null; synchronized (pos) { if (pos >= list.size()) { pos = 0; } server = list.get(pos++); } return server; }

5. Weight Random

Thought: on the basis of random, add weight;
Code implementation:

private ConcurrentMap<String, Integer> hosts = new ConcurrentHashMap<>(); private volatile Integer pos = 0; { hosts.put("127.0.0.1", 1); hosts.put("127.0.0.2", 2); hosts.put("127.0.0.3", 2); hosts.put("127.0.0.4", 1); } public String getServerRandomWeight() { List<String> list = new ArrayList<>(); for (Map.Entry<String, Integer> entry : hosts.entrySet()) { Integer value = entry.getValue(); for (int i = 0; i < value; i++) { list.add(entry.getKey()); } } Random random = new Random(); String server = list.get(random.nextInt(list.size())); return server; }

The above two ways to realize weighting are to add several times to the list, if the number of servers is large, the list will be too large; another way to realize weighting is to divide each service and weight into a segment, the larger the weight, the longer the segment

Code implementation:

private Map<String, Integer> hosts = new ConcurrentHashMap<>(); private volatile Integer pos = 0; { hosts.put("127.0.0.1", 1); hosts.put("127.0.0.2", 2); hosts.put("127.0.0.3", 2); hosts.put("127.0.0.4", 1); } public String getServerRandomWeight2() { // Accumulate all weights int sum = 0; for (Map.Entry<String, Integer> entry : hosts.entrySet()) { sum += entry.getValue(); } Random random = new Random(); int index = random.nextInt(sum); for (Map.Entry<String, Integer> entry : hosts.entrySet()) { Integer value = entry.getValue(); if(value >= index){ return entry.getKey(); } index -= entry.getValue(); } return null; }

6. Least Connections

Idea: the minimum number of connections method is to balance the load according to the current connection situation of the server. It will choose a machine with the least number of connections to provide services;
Code implementation:

// key: machine IP value: current access private Map<String, Integer> hosts = new ConcurrentHashMap<>(); private volatile Integer pos = 0; { hosts.put("127.0.0.1", 6); hosts.put("127.0.0.2", 2); hosts.put("127.0.0.3", 3); hosts.put("127.0.0.4", 8); } public String getServerLeastConnection() { // Find the minimum connection int min = 0; String key = null; for (Map.Entry<String, Integer> entry : hosts.entrySet()) { if (entry.getValue() < min) { min = entry.getValue(); key = entry.getKey(); } } hosts.put(key, min + 1); return key; }

Note: the code implementation in this paper is not suitable for the real scene, just for the simple and easy understanding of algorithm ideas;

7 May 2020, 22:18 | Views: 1825