In "Player vs. Environment" settings (PvE), tanks are nearly always at the center of combat. Tanks must do two things to be successful: first, the tank must be the party member attracting the attention of all creatures (typically by using a game mechanic that forces it to be targeted, known as threat generation). Second, the tank must work to survive incoming damage, either through a large reserve of health, or by way of damage reduction (specifically, by pure avoidance of damage, or by mitigating severity of damage via high armor values).
The first of a tank’s two responsibilities is attracting the attention of all creatures that the group is fighting. Tanks need to ensure that they remain on the top of the creature’s attack priority table at all times. This is known as 'holding aggro'. Failure to do so is the most common reason for a group wipe. There are two ways to hold aggro on creatures: high levels of threat, and taunts.
A tank must generate large amounts of threat in order to hold aggro on a target. Indeed, threat generation is the primary tool used by the tank to make themselves the most tempting or highest-priority target of enemy attacks, thereby diverting attention away from their allies.
Most tank abilities generate high amounts of threat relative to the raw numerical damage that they produce; that is, at a ratio of greater than 1:1 (in some cases, tank abilities or actions will generate threat without producing any damage at all). This is a key dynamic of threat generation, because tanks rarely produce the raw damage output needed to keep ahead on threat with allies who are playing the DPS role.
Tanks will also generate threat by attempting to interrupt casters and by apply debuffs to their targets, making the tank a higher priority target for attacks (by way of nullifying or mitigating the potential for damage), despite not dealing damage directly.
Threat is the key game mechanic for a tank. It is difficult to overstate the importance of threat for a tank:
¤ Given the choice, a Tank should ALWAYS choose to generate threat instead of damage ¤
There are very few exceptions to the above rule. Regardless of damage, so long as a tank generates more threat on their target(s) than their allies, he/she has essentially done a major portion of his/her job, and the group will usually find success.
All tanks also often have one or more taunt abilities which allow them to force a target to attack them. A taunt forces a creature to attack the tank for a set period of time, regardless of current threat levels or attack priority on the creature's attack table.
Because tanks suffer large amounts of damage, they rely on large amounts of health, Armor or evasiveness. As the tank is meant to ensure that they are the target of the current attack, as well as the next series of attacks, he/she must survive to do this.
The primary way that survival is achieved is by way of the services of the healer, who will continue to restore the tank's health so he/she does not die, and thus allowing him/her to withstand the next attack, and seek other targets.
However, the task of survivability also rests heavily on the shoulders of the tank. This is done by any combination of the three ways;
Tanks will create large health pools in order to survive the large volumes of damage that they will sustain. Because most damage is delivered in the form of a raw numerical value (as opposed to a strict percentage of the tank's health), higher health values mean that incoming damage results in a smaller percentage of health loss. Tanks can easily have twice the total health value of their non-tank allies.
Tanks will stack large amounts of armor in order to reduce the severity of the hits they sustain. Armor reduces damage by a given percentage. Note, however, that the stacking of armor is subject to diminishing returns - the addition of more and more armor will provided progressively less and less additional damage reduction as the numerical armor value of the armor rating rises.
Tanks will also achieve survivability by stacking stats and using abilities that contribute to damage avoidance. Damage avoidance refers to the act of increasing the 'miss' chance of attacks levied against the tank. A miss deals no damage at all to the tank, and thus has great value for the tank in avoiding total incoming damage over the course of a battle.
Achieving Balance In Survivability
It is a common misconception (especially among new players) that one category of survivability is universally preferable in the name of others.
Each category of survivability has an inherent weakness associated with it. Restoring large health pools can quickly depelet a healer's energy reserves. Large armor values do little to protect the tank against non-physical attacks. High avoidance can result in wild swings in incoming damage (either massive hits or complete misses, with no middle ground at all), both straining the healer and exposing the tank to death via an unluckily string of multiple 'hits' in a row.
The truth is that all survivability categories have merit. The best likelihood for surviving a fight is to balance all three mitigation categories, making the job of survivability easier, more predictable and more reliable when measured in aggregate.
There are a number of basic tank strategies which are used in tanking. Mastering these basic skills is fundamental in successful tanking.
The first of these fundamentals revolves around managing the battle area so enemies attack the tank first, rather than the weaker classes that have low-armor or low-health. Therefore, movement is a key element to tanking. Closing the distance (either by moving to a target, or instead, forcing it to move to the tank) is fundamental to managing the battlefield. This is especially true in the opening moments of a battle, when all players start at the same threat level (zero).
Area of Effect Abilities
Area of Effect (AoE) abilities (which target more than one creature) are critical to successful tanking, for a number of reasons. First, AoE abilities will generate threat on targets that are not specifically targeted by the tank. This will go a long way to maintaining aggro on all creatures, many of whom would otherwise be inclined to peel off and attack the healer (who will generate threat on all creatures whenever a heal is cast).
Second, because DPS players will tend to target creatures that are not being directly targeted by the tank (either by way of AoE abilities of their own, or by poor targeting practices), they will quickly overtake the tank’s threat level on those targets. Some of the tank's AoE abilities will help compensate for this poor play from the group(see more below).
Situational and Spatial Awareness
Tanks also need to maintain excellent situational and spatial awareness during combat. Too often, tanks get focused on one target, one objective, or one group of buttons and miss other important developments during a battle.
Situational awareness primarily involves keeping an eye on the 'bigger picture', and can range from the arrival of 'adds' (creatures who join the battle mid way through) to the damage and death of key allies (especially healers who are under attack) to changes in mob tactics at key moments of battle. A common practice for a tank is to 'cycle' through all targets in rapid succession in order to ensure that all enemies are engaged on the tank, as opposed to other group members.
Related to this is the concept of spacial awareness. This refers to being attentive to both environmental conditions (Area of Effect damage, line of sight, etc.) as well as distances between group members and mobs (staying in range of healers, gathering mobs in tight groups for effective AoE damage by DPS, etc.). More than one tank has died because he/she simply fell out of range of a healer at a critical moment.
Choosing To Be a Tank
Tanks are typically central to group play, and a large amount of responsibility is placed on the tank. Usually, a tank's death will cause creatures to overrun the party, and (as non-tanks can rarely deal with the incoming damage) the result is nearly always death for all concerned.
Fairly or not, tanks are usually blamed for player deaths and group wipes, because the most common cause of a wipe is the tank's inability to hold aggro on all creatures. While the failure to hold aggro is commonly a simple by-product of poor play by DPS players (primarily due to bad targeting practices), 'cause' and 'blame' for death does not usually go hand in hand in group play. As such, tanks must have a thick skin and an ability to explain what went on when wipes occur (as the innevitable complaints begin to arise). Well-played tanks always absorb the situation at hand at any given moment, and this allows for a careful analysis of a wipe afterwards. Remember, the best thing you can do for your party, guild, or simply your friends is to be constantly aware.
Fellow players also expect tanks to serve in the leadership role by default. While there really isn't any rational reason why a tank would be any better in the leadership role than another class, it is true that the expectation exists nonetheless. For whatever reason, a tank that doesn't have familiarity with a flashpoint, operation or boss fight is looked upon as less than competent.
Tanks also tend to be more gear dependent than other classes; that is, a tank's raw effectiveness in performing their job (threat generation and survivability) is driven to a greater extent by the quality of their gear than those serving in other roles. This is compounded by the fact that tank gear tends to be more expensive (both to purchase and to repair) than other classes.
Because of their critical role in group play and the relatively small number of tanks on a given server (compared to healer and DPS players), tanks tend to be highly sought after as team members. This can be a blessing (there is always someone who wants to team with you) or a curse (there is always someone wanting you to team with them). It is not unusual for tanks to have multiple requests to join a group within only a few minutes of logging into the game, and a calender full of operation invites.
Lastly, tanks who are not willing to continually respec between the tank and DPS roles will find solo play slower, because the while a tank's survivability is excellent, their damage output is comparatively low. This means that, while soloing, battles will take much longer to resolve. Without a leveling teammate, the increased battle time results in longer time to complete missions, and thus more time spent leveling.
Classes which can fulfill the tank role are Jedi Guardian, Jedi Shadow, Vanguard, Sith Juggernaut, Sith Assassin and Powertech. Each of these classes has a dedicated skill tree which supports tanking duties.